Method for forming positive electrode active material, method for manufacturing secondary battery, and secondary battery

ABSTRACT

A positive electrode active material with high capacity and excellent charge and discharge cycle performance, a positive electrode active material with high productivity, a positive electrode active material that suppresses a decrease in capacity, or the like is provided. Alternatively, a high-capacity secondary battery, a secondary battery with excellent charge and discharge characteristics, a highly safe or reliable secondary battery, or the like is provided. The positive electrode active material is obtained by a first heating step of heating a mixture of a first material, a second material, and a third material and a second heating step of heating a mixture which is a mixture of the mixture, a fourth material, and a fifth material and has a total amount of 15 g or more. The first material is a halogen compound including an alkali metal, the second material includes magnesium, the third material is a metal oxide including a metal A and cobalt, the fourth material includes nickel, and the fifth material includes aluminum. Each heating is performed in an atmosphere including oxygen. A temperature in the first heating step is lower than a temperature in the second heating step by 20° C. or more.

TECHNICAL FIELD

One embodiment of the present invention relates to an object, a method,or a manufacturing method. Alternatively, one embodiment of the presentinvention relates to a process, a machine, manufacture, or a composition(composition of matter). One embodiment of the present invention relatesto a semiconductor device, a display device, a light-emitting device, apower storage device, a lighting device, an electronic device, or amanufacturing method thereof. In particular, one embodiment of thepresent invention relates to a positive electrode active material thatcan be used in a secondary battery, a secondary battery, and anelectronic device including a secondary battery.

Note that in this specification, a power storage device refers to everyelement and device having a function of storing power. Examples of thepower storage device include a storage battery (also referred to as asecondary battery) such as a lithium-ion secondary battery, alithium-ion capacitor, and an electric double layer capacitor.

In addition, electronic devices in this specification mean all devicesincluding power storage devices, and electro-optical devices includingpower storage devices, information terminal devices including powerstorage devices, and the like are all electronic devices.

BACKGROUND ART

In recent years, a variety of power storage devices such as lithium-ionsecondary batteries, lithium-ion capacitors, and air batteries have beenactively developed. In particular, demand for lithium-ion secondarybatteries with high output and high energy density has rapidly grownwith the development of the semiconductor industry, for portableinformation terminals such as mobile phones, smartphones, tablets, andlaptop computers; portable music players; digital cameras; medicalequipment; next-generation clean energy vehicles (hybrid electricvehicles (HV), electric vehicles (EV), plug-in hybrid electric vehicles(PHV), and the like); and the like. The lithium-ion secondary batteriesare essential as rechargeable energy supply sources for today'sinformation society.

The performance required for lithium-ion secondary batteries includesmuch higher energy density, improved cycle performance, safety under avariety of operation environments, improved long-term reliability, andthe like.

Thus, improvement of a positive electrode active material has beenstudied to improve the cycle performance and increase the capacity oflithium-ion secondary batteries (Patent Document 1 and Patent Document2). In addition, a crystal structure of a positive electrode activematerial has also been studied (Non-Patent Document 1 to Non-PatentDocument 4).

X-ray diffraction (XRD) is one of methods used for analysis of a crystalstructure of a positive electrode active material. With the use of theICSD (Inorganic Crystal Structure Database) introduced in Non-PatentDocument 5, XRD data can be analyzed.

REFERENCES Patent Documents

-   [Patent Document 1] Japanese Published Patent Application No.    2002-216760-   [Patent Document 2] Japanese Published Patent Application No.    2006-261132

Non-Patent Documents

-   [Non-Patent Document 1] Toyoki Okumura et al., “Correlation of    lithium ion distribution and X-ray absorption near-edge structure in    O3- and O2-lithium cobalt oxides from first-principle calculation”,    Journal of Materials Chemistry, 2012, 22, pp. 17340-17348.-   [Non-Patent Document 2] Motohashi, T. et al., “Electronic phase    diagram of the layered cobalt oxide system LixCoO₂ (0.0≤x≤1.0)”,    Physical Review B, 80 (16); 165114.-   [Non-Patent Document 3] Zhaohui Chen et al., “Staging Phase    Transitions in LixCoO₂ , Journal of The Electrochemical Society,    2002, 149 (12) A1604-A1609.-   [Non-Patent Document 4] W. E. Counts et al., Journal of the American    Ceramic Society, (1953), 36 [1] 12-17. FIG. 01471.-   [Non-Patent Document 5] Belsky, A. et al., “New developments in the    Inorganic Crystal Structure Database (ICSD): accessibility in    support of materials research and design”, Acta Cryst. (2002), B58,    364-369.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

An object of one embodiment of the present invention is to provide apositive electrode active material with high capacity and excellentcharge and discharge cycle performance for a lithium-ion secondarybattery, and a forming method thereof. Another object is to provide amethod for forming a positive electrode active material with highproductivity. Another object of one embodiment of the present inventionis to provide a positive electrode active material that suppresses adecrease in capacity in charge and discharge cycles when used for alithium-ion secondary battery. Another object of one embodiment of thepresent invention is to provide a high-capacity secondary battery.Another object of one embodiment of the present invention is to providea secondary battery with excellent charge and discharge characteristics.Another object is to provide a positive electrode active material inwhich elution of a transition metal such as cobalt is inhibited evenwhen a state being charged with high voltage is held for a long time.Another object of one embodiment of the present invention is to providea highly safe or reliable secondary battery.

Another object of one embodiment of the present invention is to providea novel material, novel active material particles, a novel power storagedevice, or a forming method thereof.

Note that the descriptions of these objects do not preclude theexistence of other objects. One embodiment of the present invention doesnot have to achieve all these objects. Note that other objects can betaken from the descriptions of the specification, the drawings, and theclaims.

Means for Solving the Problems

One embodiment of the present invention is a method for forming apositive electrode active material, which includes a first step offorming a first mixture in which a first material, a second material,and a third material are mixed; a second step of heating the firstmixture to form a second mixture; a third step of forming a thirdmixture in which the second mixture, a fourth material, and a fifthmaterial are mixed; and a fourth step of heating the third mixture toform a fourth mixture. The first material is a halogen compoundincluding an alkali metal. The second material includes magnesium. Thethird material is a metal oxide including the alkali metal and cobalt.The fourth material includes nickel. The fifth material includesaluminum. The third mixture is heated in a treatment chamber of anannealing apparatus in the fourth step. A total amount of the thirdmixture heated in the treatment chamber in the fourth step is more thanor equal to 15 g. The heating in the second step is performed in anatmosphere including oxygen. The heating in the second step is performedin a temperature range higher than or equal to 600° C. and lower than orequal to 950° C. for more than or equal to 1 hour and less than or equalto 100 hours. The heating in the fourth step is performed in anatmosphere including oxygen. The heating in the fourth step is performedin a temperature range higher than or equal to 600° C. and lower than orequal to 950° C. for more than or equal to 1 hour and less than or equalto 100 hours. A temperature of the heating in the fourth step is lowerthan a temperature of the heating in the second step by 20° C. or more.

In the above-described structure, the alkali metal is preferablylithium, the first material is preferably lithium fluoride, and thesecond material is preferably magnesium fluoride.

In the above-described structure, the third material is preferablynickel hydroxide, and the fourth material is preferably aluminumhydroxide.

Another embodiment of the present invention is a method for forming apositive electrode active material, which includes a first step offorming a first mixture in which a first material, a second material, athird material, and a fourth material are mixed; and a second step ofheating the first mixture to form a second mixture. The first materialis a halogen compound including an alkali metal. The second materialincludes magnesium. The third material includes one or more selectedfrom nickel, aluminum, titanium, vanadium, and chromium. The fourthmaterial is a metal oxide including the alkali metal and cobalt. Theheating in the second step is performed in a temperature range higherthan or equal to 600° C. and lower than or equal to 950° C. for morethan or equal to 1 hour and less than or equal to 100 hours. When thefirst material, the second material, and the third material are mixedand subjected to differential scanning calorimetry, the first material,the second material, and the third material have a first peak having alocal minimum value in a range higher than or equal to 620° C. and lowerthan or equal to 920° C. The first peak is a negative peak.

In the above-described structure, the alkali metal is preferablylithium, the first material is preferably lithium fluoride, and thesecond material is preferably magnesium fluoride.

In the above-described structure, the third material preferably includesnickel, the first mixture is preferably a mixture where a fifth materialis mixed with the first material, the second material, the thirdmaterial, and the fourth material, and the fifth material preferablyincludes aluminum.

In the above-described structure, the third material is preferablynickel hydroxide.

In the above-described structure, a half width of the first peak ispreferably lower than 100° C.

In the above-described structure, a measurement temperature range of thedifferential scanning calorimetry preferably at least includes a rangehigher than or equal to 200° C. and lower than or equal to 850° C.

In the above-described structure, an atmosphere of the heating in thesecond step preferably includes oxygen.

Alternatively, one embodiment of the present invention is a method forforming a positive electrode active material, which includes a firststep of forming a first mixture in which a first material, a secondmaterial, a third material, and a fourth material are mixed and a secondstep of heating the first mixture to form a second mixture. The firstmaterial is a halogen compound including a metal A, the second materialincludes magnesium, the third material includes one or more selectedfrom nickel, aluminum, titanium, vanadium, and chromium, the fourthmaterial is a metal oxide including the metal A and cobalt, and themetal A is an alkali metal. The heating in the second step is performedin a temperature range higher than or equal to 600° C. and lower than orequal to 950° C. for more than or equal to 1 hour and less than or equalto 100 hours. When the first material, the second material, and thethird material are mixed, heated in a temperature range higher than orequal to 600° C. and lower than or equal to 950° C. for more than orequal to 1 hour and less than or equal to 100 hours, and analyzed byX-ray diffraction, the first material, the second material, and thethird material have a first diffraction peak having a local maximum at2θ of greater than or equal to 39.5° and less than or equal to 41.5°,and four peaks at 2θ=19.0°±0.25°, 2θ=31.3°±0.25°, 2θ=36.9°±0.15°, and2θ=59.4°±0.25° are not observed.

In the above-described structure, the metal A is preferably lithium, thefirst material is preferably lithium fluoride, and the second materialis preferably magnesium fluoride.

In the above-described structure, the third material preferably includesnickel, the first mixture is preferably a mixture where a fifth materialis mixed with the first material, the second material, the thirdmaterial, and the fourth material, and the fifth material preferablyincludes aluminum.

In the above-described structure, the third material is preferablynickel hydroxide.

Effect of the Invention

With one embodiment of the present invention, a positive electrodeactive material with high capacity and excellent charge and dischargecycle performance for a lithium-ion secondary battery, and a formingmethod thereof can be provided. Furthermore, a method for forming apositive electrode active material with high productivity can beprovided. Furthermore, a positive electrode active material thatsuppresses a decrease in capacity in charge and discharge cycles whenused for a lithium-ion secondary battery can be provided. Furthermore, ahigh-capacity secondary battery can be provided. Furthermore, asecondary battery with excellent charge and discharge characteristicscan be provided. Furthermore, a positive electrode active material inwhich elution of a transition metal such as cobalt is inhibited evenwhen a state being charged with high voltage is held for a long time canbe provided. Furthermore, a highly safe or reliable secondary batterycan be provided. Furthermore, a novel material, novel active materialparticles, a novel power storage device, or a forming method thereof canbe provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A, FIG. 1B, and FIG. 1C are diagrams illustrating methods forinspecting materials.

FIG. 2A, FIG. 2B, and FIG. 2C are diagrams illustrating methods forforming a positive electrode active material.

FIG. 3 is a diagram illustrating a method for forming a positiveelectrode active material.

FIG. 4 is a diagram illustrating a method for forming a positiveelectrode active material.

FIG. 5A, FIG. 5B, and FIG. 5C are diagrams illustrating a coin-typesecondary battery.

FIG. 6A, FIG. 6B, FIG. 6C, and FIG. 6D are diagrams illustratingcylindrical secondary batteries.

FIG. 7A and FIG. 7B are diagrams illustrating an example of a secondarybattery.

FIG. 8A, FIG. 8B, FIG. 8C, and FIG. 8D are diagrams illustratingexamples of secondary batteries.

FIG. 9A and FIG. 9B are diagrams illustrating examples of secondarybatteries.

FIG. 10 is a diagram illustrating an example of a secondary battery.

FIG. 11A, FIG. 11B, and FIG. 11C are diagrams illustrating a laminatedsecondary battery.

FIG. 12A and FIG. 12B are diagrams illustrating a laminated secondarybattery.

FIG. 13 is an external view of a secondary battery.

FIG. 14 is an external view of a secondary battery.

FIG. 15A, FIG. 15B, and FIG. 15C are diagrams illustrating a method formanufacturing a secondary battery.

FIG. 16A, FIG. 16B1, FIG. 16B2, FIG. 16C, and FIG. 16D are diagramsillustrating a bendable secondary battery.

FIG. 17A and FIG. 17B are diagrams illustrating a bendable secondarybattery.

FIG. 18A, FIG. 18B, FIG. 18C, FIG. 18D, FIG. 18E, FIG. 18F, FIG. 18G,and FIG. 18H are diagrams illustrating examples of electronic devices.

FIG. 19A, FIG. 19B, and FIG. 19C are diagrams illustrating examples ofelectronic devices.

FIG. 20 is a diagram illustrating examples of electronic devices.

FIG. 21A, FIG. 21B, and FIG. 21C are diagrams illustrating examples ofvehicles.

FIG. 22A, FIG. 22B, and FIG. 22C are diagrams illustrating examples ofelectronic devices.

FIG. 23A, FIG. 23B, and FIG. 23C are graphs showing results ofevaluation with DSC.

FIG. 24A and FIG. 24B are graphs showing results of evaluation with DSC.

FIG. 25 is a graph showing a result of evaluation with DSC.

FIG. 26 is a graph showing cycle performance of secondary batteries.

FIG. 27 is a graph showing XRD evaluation results.

FIG. 28 is a graph showing XRD evaluation results.

FIG. 29 is a graph showing XRD evaluation results.

FIG. 30 is a graph showing XRD evaluation results.

FIG. 31A and FIG. 31B are graphs showing cycle performance of secondarybatteries at a charge voltage of 4.60 V.

FIG. 32A and FIG. 32B are graphs showing cycle performance of secondarybatteries at a charge voltage of 4.62 V.

FIG. 33A and FIG. 33B are graphs showing cycle performance of secondarybatteries at a charge voltage of 4.64 V.

FIG. 34A and FIG. 34B are graphs showing cycle performance of secondarybatteries at a charge voltage of 4.66 V.

FIG. 35A is a graph showing cycle performance of secondary batteriesusing Sample 6.

FIG. 35B is a graph showing 1st charge and discharge curves and 50thcharge and discharge curves of the secondary battery using Sample 6 at50° C. at a charge voltage of 4.60 V.

FIG. 36A is a graph showing cycle performance of secondary batteriesusing Sample 7. FIG. 36B is a graph showing 1st charge and dischargecurves and 50th charge and discharge curves of the secondary batteryusing Sample 7 at 50° C. at a charge voltage of 4.60 V.

FIG. 37A is a graph showing cycle performance of secondary batteriesusing Sample 8. FIG. 37B is a graph showing 1st charge and dischargecurves and 50th charge and discharge curves of the secondary batteryusing Sample 8 at 50° C. at a charge voltage of 4.60 V.

FIG. 38A to FIG. 38C are graphs showing continuous chargecharacteristics of secondary batteries at a voltage of 4.60 V.

FIG. 39A to FIG. 39C are graphs showing continuous chargecharacteristics of the secondary batteries at a voltage of 4.62 V.

FIG. 40A to FIG. 40C are graphs showing continuous chargecharacteristics of the secondary batteries at a voltage of 4.64 V.

FIG. 41A to FIG. 41C are graphs showing continuous chargecharacteristics of the secondary batteries at a voltage of 4.66 V.

FIG. 42 is a graph showing endurance time in a continuous charge test.

FIG. 43A and FIG. 43B are graphs showing XRD evaluation results.

FIG. 44A and FIG. 44B are graphs showing XRD evaluation results.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention are described in detail below withreference to the drawings. Note that the present invention is notlimited to the following descriptions, and it is readily understood bythose skilled in the art that modes and details of the present inventioncan be modified in various ways. In addition, the present inventionshould not be construed as being limited to the descriptions of theembodiments below.

In addition, in this specification and the like, crystal planes andorientations are indicated by the Miller index. In the crystallography,a bar is placed over a number in the expression of crystal planes andorientations; however, in this specification and the like, crystalplanes and orientations are in some cases expressed by placing a minussign (−) before a number instead of placing the bar over the number dueto patent expression limitations. Furthermore, an individual directionwhich shows an orientation in a crystal is denoted with “[ ]”, a setdirection which shows all of the equivalent orientations is denoted with“< >”, an individual plane which shows a crystal plane is denoted with“( )”, and a set plane having equivalent symmetry is denoted with “{ }”.

In this specification and the like, segregation refers to a phenomenonin which in a solid made of a plurality of elements (e.g., A, B, and C),a certain element (e.g., B) is spatially non-uniformly distributed.

In this specification and the like, a superficial portion of a particleof an active material or the like refers to a region from a surface to adepth of approximately 10 nm. A plane generated by a crack may also bereferred to as the surface. In addition, a region in a deeper positionthan a superficial portion is referred to as an inner portion.

In this specification and the like, a layered rock-salt crystalstructure of a composite oxide including lithium and a transition metalrefers to a crystal structure in which a rock-salt ion arrangement wherecations and anions are alternately arranged is included and thetransition metal and lithium are regularly arranged to form atwo-dimensional plane, so that lithium can be two-dimensionallydiffused. Note that a defect such as a cation or anion vacancy mayexist. Moreover, in the layered rock-salt crystal structure, strictly, alattice of a rock-salt crystal is distorted in some cases.

In this specification and the like, a rock-salt crystal structure refersto a structure in which cations and anions are alternately arranged.Note that a cation or anion vacancy may exist.

In this specification and the like, a pseudo-spinel crystal structure ofa composite oxide including lithium and a transition metal belongs to aspace group R-3m, and is not a spinel crystal structure but a crystalstructure in which an ion of cobalt, magnesium, or the like iscoordinated to six oxygen atoms and the cation arrangement has symmetrysimilar to that of the spinel crystal structure. Note that in thepseudo-spinel crystal structure, a light element such as lithium issometimes coordinated to four oxygen atoms. Also in that case, the ionarrangement has symmetry similar to that of the spinel crystalstructure.

The pseudo-spinel crystal structure can also be regarded as a crystalstructure that includes Li between layers at random but is similar to aCdCl₂ type crystal structure. The crystal structure similar to the CdCl₂type crystal structure is close to a crystal structure of lithium nickeloxide when charged up to a depth of charge of 0.94 (Li_(0.06)NiO₂);however, simple and pure lithium cobalt oxide or a layered rock-saltpositive electrode active material including a large amount of cobalt isknown not to have this crystal structure generally.

Anions of a layered rock-salt crystal and anions of a rock-salt crystalhave cubic closest packed structures (face-centered cubic latticestructures). Anions of a pseudo-spinel crystal are also presumed to havecubic closest packed structures. When the pseudo-spinel crystal is incontact with the layered rock-salt crystal and the rock-salt crystal,there is a crystal plane at which orientations of cubic closest packedstructures composed of anions are aligned. Note that a space group ofthe layered rock-salt crystal and the pseudo-spinel crystal is R-3m,which is different from a space group Fm-3m of a rock-salt crystal (aspace group of a general rock-salt crystal) and a space group Fd-3m of arock-salt crystal (a space group of a rock-salt crystal having thesimplest symmetry); thus, the Miller index of the crystal planesatisfying the above conditions in the layered rock-salt crystal and thepseudo-spinel crystal is different from that in the rock-salt crystal.In this specification, a state where the orientations of the cubicclosest packed structures composed of anions in the layered rock-saltcrystal, the pseudo-spinel crystal, and the rock-salt crystal arealigned is referred to as a state where crystal orientations aresubstantially aligned in some cases.

Substantial alignment of the crystal orientations in two regions can bejudged from a TEM (transmission electron microscopy) image, a STEM(scanning transmission electron microscopy) image, a HAADF-STEM(high-angle annular dark-field scanning transmission electronmicroscopy) image, an ABF-STEM (annular bright-field scanningtransmission electron microscopy) image, or the like. X-ray diffraction(XRD), electron diffraction, neutron diffraction, and the like can alsobe used for judging. In the TEM image and the like, alignment of cationsand anions can be observed as repetition of bright lines and dark lines.When the orientations of cubic closest packed structures in the layeredrock-salt crystal and the rock-salt crystal are aligned, a state wherean angle made by the repetition of bright lines and dark lines in thelayered rock-salt crystal and the rock-salt crystal is less than orequal to 5°, further preferably less than or equal to 2.5° can beobserved. Note that in the TEM image and the like, a light element suchas oxygen or fluorine cannot be clearly observed in some cases; however,in such a case, alignment of orientations can be judged by arrangementof metal elements.

In this specification and the like, the theoretical capacity of apositive electrode active material refers to the amount of electricityfor the case where all the lithium that can be inserted and extracted inthe positive electrode active material is extracted. For example, thetheoretical capacity of LiCoO₂ is 274 mAh/g, the theoretical capacity ofLiNiO₂ is 274 mAh/g, and the theoretical capacity of LiMn₂O₄ is 148mAh/g.

In this specification and the like, the depth of charge obtained whenall the lithium that can be inserted and extracted is inserted is 0, andthe depth of charge obtained when all the lithium that can be insertedand extracted in a positive electrode active material is extracted is 1.

In addition, in this specification and the like, charging refers totransfer of lithium ions from a positive electrode to a negativeelectrode in a battery and transfer of electrons from a negativeelectrode to a positive electrode in an external circuit. For a positiveelectrode active material, extraction of lithium ions is calledcharging. A positive electrode active material with a depth of charge ofgreater than or equal to 0.7 and less than or equal to 0.9 may bereferred to as a positive electrode active material charged with a highvoltage.

Similarly, discharging refers to transfer of lithium ions from anegative electrode to a positive electrode in a battery and transfer ofelectrons from a positive electrode to a negative electrode in anexternal circuit. Discharging of a positive electrode active materialrefers to insertion of lithium ions. Furthermore, a positive electrodeactive material with a depth of charge of less than or equal to 0.06 ora positive electrode active material from which more than or equal to90% of the charge capacity is discharged from a state where the positiveelectrode active material is charged with high voltage is referred to asa sufficiently discharged positive electrode active material.

In this specification and the like, an unbalanced phase change refers toa phenomenon that causes a nonlinear change in physical quantity. Forexample, an unbalanced phase change might occur before and after peaksin a dQ/dV curve obtained by differentiating capacitance (Q) withvoltage (V) (dQ/dV), which can largely change the crystal structure.

A secondary battery includes a positive electrode and a negativeelectrode, for example. A positive electrode active material is amaterial included in the positive electrode. The positive electrodeactive material is a material that performs a reaction contributing tothe charge and discharge capacity, for example. Note that the positiveelectrode active material may partly include a material that does notcontribute to the charge and discharge capacity.

In this specification and the like, the positive electrode activematerial of one embodiment of the present invention is expressed as apositive electrode material, a secondary battery positive electrodematerial, or the like in some cases. In this specification and the like,the positive electrode active material of one embodiment of the presentinvention preferably includes a compound. In this specification and thelike, the positive electrode active material of one embodiment of thepresent invention preferably includes a composition. In thisspecification and the like, the positive electrode active material ofone embodiment of the present invention preferably includes a composite.

The discharge rate refers to the relative ratio of a current at the timeof discharging to battery capacity and is expressed in a unit C. Acurrent corresponding to 1 C in a battery with a rated capacity X (Ah)is X (A). The case where discharging is performed at a current of 2X (A)is rephrased as to perform discharging at 2 C, and the case wheredischarging is performed at a current of X/5 (A) is rephrased as toperform discharging at 0.2 C. The same applies to the charge rate; thecase where charging is performed at a current of 2X (A) is rephrased asto perform charging at 2 C, and the case where charging is performed ata current of X/5 (A) is rephrased as to perform charging at 0.2 C.

Constant current charging refers to a charging method with a fixedcharge rate, for example. Constant voltage charging refers to a chargingmethod in which voltage is fixed when reaching the upper voltage limit,for example. Constant current discharging refers to a discharging methodwith a fixed discharge rate, for example.

Embodiment 1

In this embodiment, a method for forming a positive electrode activematerial and the like according to one embodiment of the presentinvention are described.

A positive electrode active material of one embodiment of the presentinvention is obtained by mixing a halogen compound including an alkalimetal A, a compound including magnesium, and a metal oxide including thealkali metal A and a transition metal and performing annealing (alsoexpressed as heating, heat treatment, and the like in some cases). Inthe annealing process, a compound including a metal M is preferablyadded to the above-described three materials. The addition of thecompound including the metal M improves the structural stability of thepositive electrode active material and can increase the charge voltageof the secondary battery in some cases. As a result, the energy densityis increased. Moreover, the life of the secondary battery is lengthened.

In the metal oxide including the alkali metal A and the transitionmetal, the transition metal is preferably one or more of cobalt,manganese, nickel, and iron, for example. The transition metal includedin the metal oxide including the alkali metal A and the transition metalis preferably an element that is different from the metal M describedlater.

The metal oxide including the alkali metal A and the transition metalhas a layered rock-salt structure, for example. Alternatively, the metaloxide has a spinel structure, for example.

The metal M is one or more selected from nickel, aluminum, manganese,titanium, vanadium, iron, and chromium and particularly preferably oneor more of nickel and aluminum, for example.

It is suggested that a eutectic reaction occurs by mixing and heatingthe halogen compound including the alkali metal A and the compoundincluding magnesium. Alternatively, a eutectic point is lowered.Alternatively, a eutectic crystallization reaction occurs.Alternatively, a eutectic crystallizaiton point is lowered. The eutecticreaction, for example, causes melting at a temperature lower than themelting points of the materials, so that magnesium can be added to asurface and an inner portion of the metal oxide including the alkalimetal A and the transition metal.

The addition of the compound including the metal M to the threematerials, which are the halogen compound including the alkali metal A,the compound including magnesium, and the metal oxide including thealkali metal A and the transition metal, might inhibit the eutecticreaction between the halogen compound including the alkali metal A andthe compound including magnesium in some cases. A reason of inhibitingthe eutectic reaction can be a reaction of the compound including themetal M with at least one of the compound including magnesium and thehalogen compound including the alkali metal A at a temperature lowerthan the temperature at which the eutectic reaction is suggested.

The ease of the eutectic reaction may change depending on the atmosphereand pressure in annealing and the total amount of the annealed materialwith respect to the volume of the inside of a treatment chamber of anannealing apparatus.

The amount of reaction of the compound including the metal M with thecompound including magnesium and the halogen compound including thealkali metal A is preferably small at a temperature lower than thetemperature at which the eutectic reaction is suggested.

Furthermore, in the case where the compound including the metal M islikely to inhibit the eutectic reaction, the halogen compound includingthe alkali metal A, the compound including magnesium, and the metaloxide including the alkali metal A and the transition metal are mixedand subjected to annealing, and then the compound including the metal Mis mixed and annealing is performed, for example.

The reaction of the halogen compound including the alkali metal A andthe compound including magnesium with the compound including the metal Mat a temperature lower than the temperature at which the eutecticreaction is suggested can be inspected by the following method.

Note that although the compound including magnesium has been describedso far as the material that causes the eutectic reaction with thehalogen compound including the alkali metal A, a compound including anelement X can be used instead of the compound including magnesium. Asthe element X, an element such as calcium, zirconium, lanthanum, orbarium can be used. For another example, an element such as copper,potassium, sodium, or zinc can be used as the element X. Magnesium maybe included in the element X in addition to the elements describedabove. Two or more of the elements described above may be combined andused as the element X.

<DSC>

FIG. 1A illustrates an example of inspecting a reaction between amaterial 91 and a material 92 using DSC (Differential scanningcalorimetry). Here, the material 91 is the halogen compound includingthe alkali metal A, and the material 92 is the compound including theelement X.

In Step S01, the material 91 and the material 92 are prepared.

Next, in Step S02, the material 91 and the material 92 are mixed toobtain a mixture 81.

Next, in Step S03, inspection is performed. Here, DSC is performed asthe inspection. In DSC, measurement temperatures are scanned to observea change in the amount of heat. This change in the amount of heat iscaused by an endothermic reaction such as melting or an exothermicreaction such as crystallization, for example.

From the material 91 and the material 92, a change in the amount of heatsuggesting an endothermic reaction is preferably observed at atemperature T(1).

FIG. 1B illustrates an example of inspecting a reaction among thematerial 91, the material 92, and a material 93 using DSC. Here, thematerial 93 is a compound including a metal M(1). For the metal M(1),the description of the metal M can be referred to.

The mixture 81 of the material 91, the material 92, and the material 93is inspected using DSC. It is possible to inspect how much the reactionat the temperature T(1) observed in the inspection in FIG. 1A isinhibited by addition of the material 93 can be inspected.

Specifically, for example, DSC is performed at the inspection in FIG.1A, and a peak suggesting an endothermic reaction with a peak intensityI(1) is observed at the temperature T(1). Here, the temperature T(1) ispreferably higher than or equal to 620° C. and lower than or equal to920° C., further preferably higher than or equal to 700° C. and lowerthan or equal to 850° C., still further preferably higher than or equalto 700° C. and lower than or equal to 770° C.

DSC is performed at the inspection in FIG. 1B to obtain atemperature-heat flow curve. In the case where a peak suggesting anendothermic reaction is observed at an intensity that is higher than orequal to 0.3 times the peak intensity I(1) at preferably higher than orequal to [the temperature T(1)−50° C.] and lower than or equal to [thetemperature T(1)+50° C.], further preferably higher than or equal to[the temperature T(1)−30° C.] and lower than or equal to [thetemperature T(1)+30° C.], the inhibition against the eutectic reactionis judged to be insignificant. At this time, the half width of theobserved peak is preferably below 100° C., preferably lower than orequal to 50° C., further preferably lower than or equal to 30° C.

Here, the peak intensity I(1) is preferably calculated by normalizationwith the ratio of the material 91 and the material 92 in the totalweight of the mixture.

Here, the DSC scanning rate is 20° C./min. for example, preferablyhigher than or equal to 2° C./min. and lower than or equal to 30°C./min. for example.

Furthermore, the peak intensity may be a difference in height between alocal maximum point and a local minimum point observed in an obtainedDSC differential waveform around a temperature at which a peak isobserved in a temperature-heat flow curve before the differentiation.The absolute value of the difference between the peak position where thelocal maximum point of the differential waveform is observed and thepeak position of the temperature-heat flow curve is preferably less than0.5 times the half width of the peak of the temperature-heat flow curve.The absolute value of the difference between the peak position where thelocal minimum point of the differential waveform is observed and thepeak position of the temperature-heat flow curve is also preferably lessthan 0.5 times the half width of the peak of the temperature-heat flowcurve.

FIG. 1C illustrates an example of inspecting a reaction among thematerial 91, the material 92, the material 93, and a material 94 usingDSC. Here, the material 94 is a compound including a metal M(2). For themetal M(2), the description of the metal M can be referred to.Furthermore, the metal M(2) preferably includes a metal that isdifferent from the metal M(1).

The mixture 81 of the material 91, the material 92, the material 93, andthe material 94 is inspected using DSC. It is possible to inspect howmuch the reaction at the temperature T(1) observed in the inspection inFIG. 1A is inhibited by addition of the material 93 and the material 94.In the case where the inhibition against the reaction at the temperatureT(1) is suggested, the material 94 is used instead of the material 93 inFIG. 1B to perform inspection. Thus, it can be found which of thematerial 93 and the material 94 contributes more to the inhibitionagainst the reaction at the temperature T(1).

In Step S01 in FIG. 1A, FIG. 1B, and FIG. 1C, a metal oxide 95 may beadded to perform inspection. Here, the metal oxide 95 is the metal oxideincluding the alkali metal A and the transition metal.

In the case where DSC is performed with the addition of the metal oxide95, the temperature T(1) at which the peak is observed might becomehigher than that of the case where the metal oxide 95 is not added byapproximately 100° C., for example.

As the alkali metal A, lithium, sodium, potassium, or the like may beused, for example, and lithium is particularly preferably used. As themetal oxide including the alkali metal A and the transition metal, ametal oxide having a layered rock-salt structure may be used, forexample. Alternatively, a metal oxide having a structure represented bya space group R-3m may be used.

As the metal oxide including the alkali metal A and the transitionmetal, lithium cobalt oxide, lithium manganite, lithium nickel oxide,lithium cobalt oxide where manganese substitutes for part of cobalt,lithium cobalt oxide where nickel substitutes for part of cobalt, ornickel-manganese-lithium cobalt oxide can be used.

In the case where the metal oxide including the alkali metal A and thetransition metal includes nickel, the proportion of nickel atoms (Ni) inthe sum of cobalt atoms and nickel atoms (Co+Ni) (Ni/(Co+Ni)) ispreferably less than 0.1, further preferably less than or equal to0.075, for example. When a state being charged with high voltage is heldfor a long time, the transition metal dissolves in an electrolytesolution from the positive electrode active material, and the crystalstructure might be broken. However, when nickel is included at theabove-described proportion, dissolution of the transition metal from apositive electrode active material 100 can be inhibited in some cases.

Examples of the halogen compound including the alkali metal A includelithium fluoride, sodium fluoride, potassium fluoride, lithium chloride,sodium chloride, and calcium chloride. In particular, lithium fluorideis preferable because it is easily melted in an annealing processdescribed later.

As the compound including the element X, a compound including magnesiumcan be used. Examples of the compound including magnesium includemagnesium fluoride, magnesium oxide, magnesium hydroxide, magnesiumcarbonate, and magnesium chloride.

In forming the positive electrode active material of one embodiment ofthe present invention, lithium fluoride is preferably used as thehalogen compound including the alkali metal A, and magnesium fluoride ispreferably used as the compound including magnesium. By mixing lithiumfluoride, melting can be caused at a temperature lower than the meltingpoint of magnesium fluoride, and a positive electrode active material isformed utilizing this eutectic phenomenon.

As the compound including the metal M, a hydroxide, an oxide, or thelike of a metal can be used, for example. In the case where a metalincluded in the metal M is nickel, nickel hydroxide, nickel oxide, orthe like can be used, for example. In the case where a metal included inthe metal M is aluminum, aluminum hydroxide, aluminum oxide, or the likecan be used, for example. In the case where a metal included in themetal M is manganese, manganese hydroxide, manganese oxide, or the likecan be used, for example.

Furthermore, as the compound including the metal M, a metal alkoxide maybe used. For example, aluminum isopropoxide, tetramethoxy titanium, orthe like can be used.

The positive electrode active material of one embodiment of the presentinvention preferably includes the above-described metal oxide includingthe alkali metal A and the transition metal. In the case where thepositive electrode active material of one embodiment of the presentinvention includes particles, the particles preferably include theabove-described metal oxide including the alkali metal A and thetransition metal, for example.

The positive electrode active material of one embodiment of the presentinvention preferably includes magnesium and the metal M. Furthermore,the positive electrode active material of one embodiment of the presentinvention includes positive electrode active material particles of oneembodiment of the present invention; in the case where the particlesinclude the above-described metal oxide including the alkali metal A andthe transition metal, the metal oxide includes at least one of magnesiumand the metal M in a region, specifically in surfaces and the vicinityof the particles, for example.

<XRD>

XRD may be performed as the inspection illustrated in FIG. 1B or FIG.1C. For example, in the case where XRD suggests the existence of acompound which includes the element X included in the material 92 andone or more of the metal elements included in the material 93 and/or thematerial 94, it is judged that the eutectic reaction between thematerial 91 and the material 92 is significantly inhibited. Note that inthe inspection, heating is performed first in a temperature range higherthan or equal to 600° C. and lower than or equal to 950° C. for morethan or equal to 1 hour and less than or equal to 100 hours, and thenXRD evaluation is performed.

In the case where the metal oxide including the alkali metal A and thetransition metal has a layered rock-salt structure, it is judged thatthe eutectic reaction is significantly inhibited when a peak derivedfrom a spinel structure is observed in XRD, for example. Morespecifically, for example, in the case where aluminum is used as themetal M, it is judged that the eutectic reaction is significantlyinhibited when at least any of four peaks at 2θ=19.0°±0.25°,2θ=31.3°±0.25°, 2θ=36.9°±0.15°, and 2θ=59.4°±0.25° is observed in XRD.It is suggested that these peaks are derived from MgAl₂O₄. These peaksare preferably not observed or preferably have sufficiently low peakintensities, for example. For example, it is not preferable that all thefour peaks suggesting MgAl2O4 at 2θ=19.0°±0.25°, 2θ=31.3°±0.25°,2θ=36.9°±0.15°, and 2θ=59.4°±0.25° be observed having intensities thatare more than or equal to 0.02 times the intensity of the peak havingthe highest intensity among the peaks observed in a 2θ range higher thanor equal to 15° and lower than or equal to 90° in XRD.

Here, even in the case where the material 91, the material 92, and atleast one of the material 93 and the material 94 are mixed, heated, andsubjected to XRD analysis to observe a peak from which it is judged thatthe eutectic reaction is significantly inhibited, if the metal oxide 95is further mixed with the material 91, the material 92, and at least oneof the material 93 and the material 94 and heating is performed, thepeak from which it is judged that the eutectic reaction is significantlyinhibited might not be observed because the intensity of a peak derivedfrom the metal oxide 95 is high.

<Formation Method>

FIG. 2A, FIG. 2B, and FIG. 2C illustrate examples of a method forforming a positive electrode active material of one embodiment of thepresent invention using the material 91, the material 92, the material93, and the material 94, which are described with reference to FIG. 1,and the metal oxide 95.

Note that some steps in a formation procedure described in thisspecification and the like are sometimes not illustrated for simplicity.

In a procedure illustrated in FIG. 2A, the material 91, the material 92,the material 93, the material 94, and the metal oxide 95 are preparedand mixed in Step S11, annealing is performed in Step S34, and thepositive electrode active material 100 is obtained in Step S36.

In the case where it is judged in the inspection in FIG. 1B or FIG. 1Cthat at least one of the material 93 and the material 94 significantlyinhibits the reaction suggesting the eutectic reaction between thematerial 91 and the material 92, it is preferable to use the procedureillustrated in FIG. 2B to obtain the positive electrode active material100. In particular, in the case where the total amount of the annealedmaterial is large, using the procedure illustrated in FIG. 2B issometimes preferable for more uniform processing. More uniformprocessing can improve the quality of the positive electrode activematerial 100 to be obtained. Specifically, for example, in the casewhere the material is in the form of powder and the total amount of thepowder is more than or equal to 15 g, the procedure illustrated in FIG.2B is preferably used. In the case where the total amount of the powderis more than or equal to 15 g for example, a surface of the powder isnot fully exposed to an annealing atmosphere by one-time annealing, insome cases. In that case, the eutectic reaction is more likely to beinhibited in some cases. Using the procedure in FIG. 2B is preferablebecause it can more surely cause the eutectic reaction. Alternatively,the atmosphere and pressure of the annealing and the total amount of theannealed material with respect to the volume of a treatment chamber ofan annealing apparatus may be adjusted. In contrast, in the case wherethe total amount of the powder is less than 15 g, the surface of thepowder is likely to be exposed to the annealing atmosphere and theinhibition against the eutectic reaction is suppressed in some cases.Here, annealing is performed with a heating furnace, for example. Thevolume of the heating furnace is for example more than or equal to 10 L,more than or equal to 20 L, or more than or equal to 30 L.

The procedure illustrated in FIG. 2B is a procedure where the material93 and the material 94 are added after annealing in Step S34 isperformed. The material 91, the material 92, and the metal oxide 95 areprepared and mixed in Step S11, and annealing is performed in Step S34.The material 93 and the material 94 are added to and mixed with amixture obtained in Step S34, annealing is performed in Step S55, andthe positive electrode active material 100 is obtained in Step S36.

With the procedure illustrated in FIG. 2B, the inhibition against theeutectic reaction between the material 91 and the material 92 by thematerial 93 and the material 94 is suppressed.

In the case where it is judged that the material 94 significantlyinhibits the reaction suggesting the eutectic reaction between thematerial 91 and the material 92 and that the material 93 does notsignificantly inhibit the reaction, a procedure illustrated in FIG. 2Ccan be used, for example.

In the procedure illustrated in FIG. 2C, the material 91, the material92, the material 93, and the metal oxide 95 are prepared and mixed inStep S11, and annealing is performed in Step S34. The material 94 isadded to and mixed with a mixture obtained in Step S34, annealing isperformed in Step S55, and the positive electrode active material 100 isobtained in Step S36.

<Formation Method 2>

FIG. 3 illustrates an example of the formation method illustrated inFIG. 2B. The formation method illustrated in FIG. 3 is an example of thecase where a metal oxide including an alkali metal and cobalt is used asthe metal oxide including the alkali metal A and the transition metal.In addition, illustrated is an example of the case where a compoundincluding magnesium is used as the compound including the element X.

<Step S11>

As illustrated in Step S11 in FIG. 3, materials of a mixture 902 areprepared first. In FIG. 3, lithium fluoride LiF is prepared as thehalogen compound including the alkali metal A, and magnesium fluorideMgF₂ is prepared as the compound including magnesium. When lithiumfluoride LiF and magnesium fluoride MgF₂ are mixed at approximatelyLiF:MgF₂=65: 35 (molar ratio), the effect of decreasing the meltingpoint of the mixture becomes the highest (Non-Patent Document 4). On theother hand, when the amount of lithium fluoride increases, cycleperformance might deteriorate because of a too large amount of lithium.Therefore, the molar ratio of lithium fluoride LiF to magnesium fluorideMgF₂ is preferably LiF:MgF₂=x:1 (0≤x≤1.9), further preferablyLiF:MgF₂=x: 1 (0.1≤x≤0.5), still further preferably LiF:MgF₂=x:1 (x=thevicinity of 0.33). Note that in this specification and the like, thevicinity means a value greater than 0.9 times and smaller than 1.1 timesa certain value.

In addition, in the case where the following mixing and grinding step isperformed by a wet process, a solvent is prepared. As the solvent,ketone such as acetone; alcohol such as ethanol or isopropanol; ether;dioxane; acetonitrile; N-methyl-2-pyrrolidone (NMP); or the like can beused. An aprotic solvent that hardly reacts with lithium is furtherpreferably used. In Step S11 in FIG. 3, acetone is used.

<Step S12>

Next, in Step S12, the materials of the mixture 902 are mixed andground. Although the mixing can be performed by a dry process or a wetprocess, the wet process is preferable because the materials can beground to smaller size. For example, a ball mill, a bead mill, or thelike can be used for the mixing. When the ball mill is used, a zirconiaball is preferably used as media, for example. The mixing and grindingstep is preferably performed sufficiently to pulverize the mixture 902.

<Step S13, Step S14>

Next, in Step S13, the materials mixed and ground in the above mannerare collected, whereby the mixture 902 is obtained in Step S14.

For example, the D50 of the mixture 902 is preferably greater than orequal to 600 nm and less than or equal to 20 μm, further preferablygreater than or equal to 1 μm and less than or equal to 10 μm. Whenmixed with a composite oxide including lithium, a transition metal, andoxygen in a later step, the mixture 902 pulverized to such a small sizeis easily attached to surfaces of composite oxide particles uniformly.The mixture 902 is preferably attached to the surfaces of the compositeoxide particles uniformly in order that both halogen and magnesium areeasily distributed to the superficial portion of the composite oxideparticles after heating. When there is a region including neitherhalogen nor magnesium in the superficial portion, the positive electrodeactive material might be less likely to have a pseudo-spinel crystalstructure, which is to be described later, in the charged state.

<Step S25>

Next, in Step S25, the metal oxide 95 is prepared as the metal oxideincluding the alkali metal A and cobalt. The metal oxide 95 can beobtained by baking a mixture of a material including the alkali metal Aand a material including cobalt. Alternatively, a metal oxidesynthesized in advance may be used.

<Step S31>

Next, in Step S31, the mixture 902 and the metal oxide 95 are mixed. Theratio of the number of cobalt atoms TM in the metal oxide 95 to thenumber of magnesium atoms MgMix1 included in the mixture 902 ispreferably TM:MgMix1=1:y (0.005≤y≤0.05), further preferablyTM:MgMix1=1:y (0.007≤y≤0.04), still further preferably approximatelyTM:MgMix1=1:0.02.

The condition of the mixing in Step S31 is preferably milder than thatof the mixing in Step S12 in order not to damage the particles of themetal oxide 95. For example, a condition with a lower rotation frequencyor shorter time than the mixing in Step S12 is preferable. In addition,it can be said that the dry process has a milder condition than the wetprocess. For example, a ball mill, a bead mill, or the like can be usedfor the mixing. When the ball mill is used, a zirconia ball ispreferably used as media, for example.

<Step S32, Step S33>

Next, in Step S32, the materials mixed in the above manner arecollected, whereby a mixture 903 is obtained in Step S33.

<Step S34>

Next, in Step S34, the mixture 903 is annealed.

The annealing is preferably performed at an appropriate temperature foran appropriate time. The appropriate temperature and time vary dependingon conditions such as the particle size and the composition of the metaloxide 95. In the case where the particle size is small, the annealing ispreferably performed at a lower temperature or for a shorter time thanthe case where the particle size is large, in some cases.

When the average particle diameter (D50) of the particles in Step S25 isapproximately 12 μm, for example, the annealing temperature ispreferably higher than or equal to 600° C. and lower than or equal to950° C., for example. The annealing time is preferably longer than orequal to 3 hours, further preferably longer than or equal to 10 hours,still further preferably longer than or equal to 60 hours, for example.

When the average particle diameter (D50) of the particles in Step S25 isapproximately 5 μm, the annealing temperature is preferably higher thanor equal to 600° C. and lower than or equal to 950° C., for example. Theannealing time is preferably longer than or equal to 1 hour and shorterthan or equal to 10 hours, further preferably approximately 2 hours, forexample.

The temperature decreasing time after the annealing is, for example,preferably longer than or equal to 10 hours and shorter than or equal to50 hours.

It is considered that when the mixture 903 is annealed, a materialhaving a lower melting point (e.g., lithium fluoride with a meltingpoint of 848° C.) in the mixture 902 is melted first and distributed toa superficial portion of the particles of the metal oxide 95. Next, theexistence of the melted material presumably causes a decrease of themelting points of other materials, resulting in melting of the othermaterials. For example, magnesium fluoride (melting point: 1263° C.) ispresumably melted and distributed to the superficial portion of theparticles of the metal oxide 95.

Elements included in the mixture 902 distributed to the superficialportion probably form a solid solution in the particles of the metaloxide 95.

The elements included in the mixture 902 are diffused faster in thesuperficial portion and the vicinity of the grain boundary than in theinner portion of the particles of the metal oxide 95. Therefore, theconcentrations of magnesium and halogen in the superficial portion andthe vicinity of the grain boundary are higher than those of magnesiumand halogen in the inner portion. As described later, the higher themagnesium concentration in the superficial portion and the vicinity ofthe grain boundary is, the more effectively the change in the crystalstructure can be inhibited.

<Step S35, Step S36>

Next, in Step S35, the materials annealed in the above manner arecollected, whereby a mixture 904 is obtained in Step S36.

<Step S41>

Next, in Step S41, a metal M source is prepared. When the metal M isaluminum, for example, the molar concentration of aluminum included inthe metal source ranges from 0.001 to 0.02 times that of cobalt with thenumber of cobalt atoms included in lithium cobalt oxide regarded as 1.When the metal M is nickel, for example, the molar concentration ofnickel included in the metal source ranges from 0.001 to 0.02 times thatof cobalt with the number of cobalt atoms included in lithium cobaltoxide regarded as 1. When the metal M is aluminum and nickel, forexample, the molar concentration of aluminum included in the metalsource ranges from 0.001 to 0.02 times that of cobalt and the molarconcentration of nickel included in the metal source ranges from 0.001to 0.02 times that of cobalt with the number of cobalt atoms included inlithium cobalt oxide regarded as 1.

In the case where mixing is performed by a wet process in subsequentStep S42, a solvent is prepared also in Step S41.

In Step S41 in FIG. 3, an example of using nickel hydroxide as the metalsource and acetone as the solvent is shown.

<Step S42>

Next, in Step S42, the metal source and the solvent are mixed andground. For the mixing and grinding, the conditions in Step S12 or thelike can be referred to.

<Step S43>

Next, in Step S43, the metal M source ground in Step S42 is collected.

<Step S44>

Next, in Step S44, a metal M source including a metal that is differentfrom the metal included in the metal M source prepared in Step S41 isprepared. In the case where mixing is performed by a wet process insubsequent Step S45, a solvent is prepared also in Step S44. In Step S44in FIG. 3, as an example, aluminum hydroxide is prepared as the metalsource, and acetone is prepared as the solvent.

<Step S45>

Next, in Step S45, the metal source and the solvent are mixed andground. For the mixing and grinding, the conditions in Step S12 or thelike can be referred to.

<Step S46>

Next, in Step S46, the metal M source ground in Step 45 is collected.

<Step S53>

Next, in Step S53, the mixture 904, the metal M source collected in StepS43, and the metal M source collected in Step S46 are mixed.

<Step S54, Step S55>

Next, in Step S54, the mixture is collected, whereby a mixture 905 isobtained in Step S55.

<Step S56>

Next, in Step S56, the mixture 905 is annealed. For the annealing time,the retention time at a temperature within a specified range ispreferably longer than or equal to 1 hour and shorter than or equal to50 hours, further preferably longer than or equal to 2 hours and shorterthan or equal to 20 hours. When the baking time is too short, thecrystallinity of a compound including the metal M formed in thesuperficial portion is low in some cases. Alternatively, the metal M isnot sufficiently diffused in some cases. Alternatively, an organicsubstance may remain on the surface in some cases. However, when thebaking time is too long, the metal M is diffused too much so that theconcentration at the superficial portion and the vicinity of the crystalgrain boundary might be low. Furthermore, the productivity is lowered.

The specified temperature is preferably higher than or equal to 500° C.and lower than or equal to 1200° C., further preferably higher than orequal to 700° C. and lower than or equal to 920° C., and still furtherpreferably higher than or equal to 800° C. and lower than or equal to900° C. When the specified temperature is too low, the crystallinity ofthe compound including the metal M formed in the superficial portion islow in some cases. Alternatively, the metal M is not sufficientlydiffused in some cases. Alternatively, an organic substance may remainon the surface in some cases.

The baking is preferably performed in an oxygen-containing atmosphere.In the case where the oxygen partial pressure is low, cobalt might bereduced unless the baking temperature is lowered.

In this embodiment, the specified temperature is 850° C. and kept for 2hours, the temperature rising rate is 200° C./h, and the flow rate ofoxygen is 10 L/min.

The cooling time after the baking is preferably long, in which case acrystal structure is easily stabilized. For example, the temperaturedecreasing time from the specified temperature to room temperature ispreferably longer than or equal to 10 hours and shorter than or equal to50 hours. Here, the baking temperature in Step S56 is preferably lowerthan the baking temperature in Step S34. For example, the bakingtemperature in Step S56 is preferably lower than the baking temperaturein Step S34 by 20° C. or more, 30° C. or more, or 45° C. or more.

<Step S57, Step S58>

Next, in Step S57, the cooled particles are collected. Moreover, theparticles are preferably made to pass through a sieve. Through theabove-described process, the positive electrode active material 100 isobtained in Step S58.

<Formation Method 3>

Next, FIG. 4 illustrates an example of the formation method illustratedin FIG. 2C. The formation method illustrated in FIG. 4 is an example ofthe case where a metal oxide including an alkali metal and cobalt isused as the metal oxide including the alkali metal A and the transitionmetal. In addition, illustrated is an example of the case where acompound including magnesium is used as the compound including theelement X.

<Step S11>

Step S11 illustrated in FIG. 4 is different from that in FIG. 3 inpreparing a metal M(1) source in addition to the halogen compoundincluding the alkali metal A, the compound including magnesium, and thesolvent.

<Step S12 to Step S14>

Next, through Step S12, Step S13, and Step S14, a mixture 906, which isa mixture of the halogen compound including the alkali metal A, thecompound including magnesium, and the metal M(1) source, is obtained.For the conditions and the like of Step S12 to Step S14, the descriptionof FIG. 3 can be referred to.

After Step S14, the mixture 906 may be inspected.

<Step S25>

Next, in Step S25, the metal oxide 95 is prepared as the metal oxideincluding the alkali metal A and cobalt. The metal oxide 95 can beobtained by baking a mixture of a material including the alkali metal Aand a material including cobalt. Alternatively, a metal oxidesynthesized in advance may be used.

<Step S31 to Step S33>

Next, through Step S31, Step S32, and Step S33, a mixture 907, which isa mixture of the mixture 906 and the metal oxide 95, is obtained. Forthe conditions and the like of Step S31 to Step S33, the description ofFIG. 3 can be referred to.

After Step S33, the mixture 907 may be inspected.

<Step S34>

Next, in Step S34, the mixture 907 is annealed. For the conditions andthe like of the annealing, the description of FIG. 3 can be referred to.

<Step S35>

Next, in Step S35, the annealed powder is collected, whereby a mixture908 is obtained in Step S36.

<Step S47>

Next, in Step S47, the metal M(2) source and a solvent are prepared.Here, an example of employing a sol-gel method using aluminumisopropoxide as the metal M(2) source and isopropanol as the solvent isshown.

<Step S53>

Next, in Step S53, aluminum isopropoxide is dissolved in isopropanol,and the mixture 908 is further mixed therein. The required amount of themetal alkoxide varies depending on the particle size of the metal oxide95. If the particle diameter (D50) is approximately 20 μm, aluminumisopropoxide is preferably added so that the concentration of aluminumincluded in aluminum isopropoxide ranges from 0.001 to 0.02 times thatof cobalt with the number of cobalt atoms included in the metal oxide 95regarded as 1. The mixture 908 is preferably stirred in an atmospherecontaining water vapor. The stirring can be performed with a magneticstirrer, for example. The stirring is performed for a time long enoughfor water and metal alkoxide in the atmosphere to cause hydrolysis andpolycondensation reaction. For example, the stirring can be performed at25° C. and a humidity of 90% RH (Relative Humidity) for 4 hours.Alternatively, the stirring may be performed under an atmosphere wherethe humidity and temperature are not adjusted, for example, an airatmosphere in a fume hood. In such a case, the stirring time ispreferably set longer and can be 12 hours or longer at room temperature,for example.

Reaction between moisture and metal alkoxide in the atmosphere enables asol-gel reaction to proceed more slowly as compared with the case whereliquid water is added. Alternatively, reaction between metal alkoxideand water at room temperature enables a sol-gel reaction to proceed moreslowly as compared with the case where heating is performed at atemperature higher than the boiling point of alcohol serving as asolvent, for example. A sol-gel reaction that proceeds slowly enablesformation of a high-quality coating layer with a uniform thickness.

<Step S54, Step S55>

Next, in Step S54, a precipitate is collected from the mixed solution,whereby a mixture 909 is obtained in Step S55. As the collection method,filtration, centrifugation, evaporation to dryness, or the like can beused. The precipitate can be washed with alcohol that is the same as thesolvent in which the metal alkoxide is dissolved. Then, the collectedresidue is dried to obtain the mixture 909. In the drying step, vacuumor ventilation drying can be performed at 80° C. for 1 hour to 4 hours,for example.

<Step S56>

Next, in Step S56, the mixture 909 is baked. For the baking time, theretention time at a temperature within a specified range is preferablylonger than or equal to 1 hour and shorter than or equal to 50 hours,further preferably longer than or equal to 2 hours and shorter than orequal to 20 hours. When the baking time is too short, the crystallinityof a compound including the metal M(2) formed in the superficial portionis low in some cases. Alternatively, the metal M(2) is not sufficientlydiffused in some cases. Alternatively, an organic substance may remainon the surface in some cases. However, when the baking time is too long,the metal M(2) is diffused too much so that the concentration at thesuperficial portion and the vicinity of the crystal grain boundary mightbe low. Furthermore, the productivity is lowered.

The specified temperature is preferably higher than or equal to 500° C.and lower than or equal to 1200° C., further preferably higher than orequal to 700° C. and lower than or equal to 920° C., and still furtherpreferably higher than or equal to 800° C. and lower than or equal to900° C. When the specified temperature is too low, the crystallinity ofthe compound including the metal M(2) formed in the superficial portionis low in some cases. Alternatively, the metal M(2) is not sufficientlydiffused in some cases. Alternatively, an organic substance may remainon the surface in some cases.

The baking is preferably performed in an oxygen-containing atmosphere.In the case where the oxygen partial pressure is low, Co might bereduced unless the baking temperature is lowered.

In this embodiment, the specified temperature is 850° C. and kept for 2hours, the temperature rising rate is 200° C./h, and the flow rate ofoxygen is 10 L/min.

The cooling time after the baking is preferably long, in which case acrystal structure is easily stabilized. For example, the temperaturedecreasing time from the specified temperature to room temperature ispreferably longer than or equal to 10 hours and shorter than or equal to50 hours. Here, the baking temperature in Step S56 is preferably lowerthan the baking temperature in Step S34.

<Step S57, Step S58>

Next, in Step S57, the cooled particles are collected. Moreover, theparticles are preferably made to pass through a sieve. Through theabove-described process, the positive electrode active material 100 isobtained in Step S58.

Although the method for forming a positive electrode active materialwhich includes the metal M, the material 93, the material 94, and nickelor aluminum is described with reference to FIG. 2 to FIG. 4, oneembodiment of the present invention is not limited thereto. As describedin the beginning of this embodiment, a positive electrode activematerial of one embodiment of the present invention is obtained bymixing a halogen compound including an alkali metal A, a compoundincluding magnesium, and a metal oxide including the alkali metal A anda transition metal and performing annealing (also expressed as heating,heat treatment, and the like in some cases). The metal M is notnecessarily included. In the case where the metal M is not included, thenumber of annealing processes is the most preferably one, in some cases.In the case where the number of annealing processes is one, theproductivity can be increased as compared with the case where the numberof annealing processes is more than one.

This embodiment can be used in appropriate combination with the otherembodiments.

Embodiment 2

In this embodiment, a positive electrode active material of oneembodiment of the present invention is described.

<Positive Electrode Active Material>

With the use of the positive electrode active material of one embodimentof the present invention, the capacity of a secondary battery isincreased and a reduction in discharge capacity due to charge anddischarge cycles can be inhibited.

[Structure of Positive Electrode Active Material]

The positive electrode active material preferably includes a metal(hereinafter, an element A) serving as a carrier ion. As the element A,an alkali metal such as lithium, sodium, or potassium or a Group 2element such as calcium, beryllium, or magnesium can be used, forexample.

In the positive electrode active material, carrier ions are releasedfrom the positive electrode active material in charging. A larger amountof the released element A means a larger amount of ions contributing tothe capacity of a secondary battery, increasing the capacity. However, alarge amount of the released element A easily causes a collapse of thecrystal structure of a compound included in the positive electrodeactive material. The collapse of the crystal structure of the positiveelectrode active material sometimes decreases the discharge capacity dueto charge and discharge cycles. When the positive electrode activematerial of one embodiment of the present invention includes the elementX, the collapse of the crystal structure when carrier ions are releasedin charging the secondary battery can be inhibited in some cases. Forexample, the element X is partly substituted at the element A positions.An element such as magnesium, calcium, zirconium, lanthanum, or bariumcan be used as the element X. For another example, an element such ascopper, potassium, sodium, or zinc can be used as the element X. Two ormore of the elements described above may be combined and used as theelement X.

Furthermore, the positive electrode active material of one embodiment ofthe present invention preferably includes halogen in addition to theelement X. The positive electrode active material preferably includeshalogen such as fluorine or chlorine. When the positive electrode activematerial of one embodiment of the present invention includes thehalogen, substitution of the element X at the element A position ispromoted in some cases.

The positive electrode active material of one embodiment of the presentinvention includes a metal (hereinafter, an element Me) whose valencenumber changes due to charge and discharge of a secondary battery. Theelement Me is a transition metal, for example. The positive electrodeactive material of one embodiment of the present invention includes oneor more of cobalt, nickel, and manganese, particularly cobalt, as theelement Me, for example. The positive electrode active material mayinclude, at an element Me position, an element that has no valencechange and can have the same valence as the element Me, such asaluminum, specifically, a trivalent representative element, for example.The element X may be substituted at the element Me position, forexample. In the case where the positive electrode active material of oneembodiment of the present invention is an oxide, the element X may besubstituted at an oxygen position.

A lithium composite oxide having a layered rock-salt crystal structureis preferably used as the positive electrode active material of oneembodiment of the present invention, for example. More specifically,lithium cobalt oxide, lithium nickel oxide, a lithium composite oxidecontaining nickel, manganese, and cobalt, a lithium composite oxidecontaining nickel, cobalt, and aluminum, or the like can be used as thelithium composite oxide having a layered rock-salt crystal structure,for example. Moreover, each of these positive electrode active materialsis preferably represented by a space group R-3m.

In a positive electrode active material having a layered rock-saltcrystal structure, the crystal structure is disordered with increasingdepth of charge in some cases. Here, the disorder of the crystalstructure means deviation in layers, for example. In the case where thedisorder of the crystal structure is irreversible, the capacity of thesecondary battery might be reduced with repeated charge and discharge.

When the positive electrode active material of one embodiment of thepresent invention includes the element X, for example, the deviation inlayers is suppressed even when the depth of charge is increased. Bysuppressing the deviation, a volume change in charging and dischargingcan be reduced. Accordingly, the positive electrode active material ofone embodiment of the present invention can have excellent cycleperformance. In addition, the positive electrode active material of oneembodiment of the present invention can have a stable crystal structurein a high-voltage charged state. Thus, in the positive electrode activematerial of one embodiment of the present invention, a short circuit issometimes unlikely to occur while the high-voltage charged state ismaintained. This is preferable because the safety is further improved.

The positive electrode active material of one embodiment of the presentinvention has a small crystal-structure change and a small volumedifference per the same number of transition metal atoms between asufficiently discharged state and a high-voltage charged state.

The positive electrode active material of one embodiment of the presentinvention is expressed by a chemical formula AM_(y)O_(z) (y>0, z>0) insome cases. For example, lithium cobalt oxide is expressed as LiCoO₂ insome cases. For another example, lithium nickel oxide is expressed asLiNiO₂ in some cases.

In the case where the depth of charge is greater than or equal to 0.8,in some cases, the positive electrode active material of one embodimentof the present invention, which includes the element X, is representedby a space group R-3m and has not the spinel crystal structure but astructure in which an ion of the element Me (e.g., cobalt), the elementM (e.g., magnesium), or the like is coordinated to six oxygen atoms andthe cation arrangement has symmetry similar to that of the spinelcrystal structure. This structure is referred to as the pseudo-spinelcrystal structure in this specification and the like. Note that in thepseudo-spinel crystal structure, a light element such as lithium iscoordinated to four oxygen atoms in some cases; also in that case, theion arrangement has symmetry similar to that of the spinel crystalstructure.

The structure of the positive electrode active material becomes unstabledue to release of carrier ions by charging. It can be said that thepseudo-spinel crystal structure is capable of maintaining high stabilityeven after release of carrier ions.

In the case where the depth of charge is high in the present inventionand when the positive electrode active material having the pseudo-spinelstructure is used for the secondary battery, the structure of thepositive electrode active material is stable for example at a voltage ofapproximately 4.6 V, preferably at a voltage of approximately 4.62 V to4.7 V, with reference to the potential of lithium metal, and a reductionin capacity due to charging and discharging can be suppressed. Note thatin the case of using graphite as the negative electrode active materialin the secondary battery, for example, the structure of the positiveelectrode active material is stable for example at a voltage of thesecondary battery of higher than or equal to 4.3 V and lower than orequal to 4.5 V, preferably at a voltage of higher than or equal to 4.35V and lower than or equal to 4.55 V, and a reduction in capacity due tocharging and discharging can be suppressed.

The pseudo-spinel crystal structure can also be regarded as a crystalstructure that includes Li between layers at random but is similar to aCdCl₂ type crystal structure. The crystal structure similar to the CdCl₂type crystal structure is close to a crystal structure of lithium nickeloxide when charged up to a depth of charge of 0.94 (Li_(0.06)NiO₂);however, pure lithium cobalt oxide or a layered rock-salt positiveelectrode active material containing a large amount of cobalt is knownnot to have this crystal structure generally.

Anions of a layered rock-salt crystal and anions of a rock-salt crystalhave cubic closest packed structures (face-centered cubic latticestructures). Anions of a pseudo-spinel crystal are also presumed to havecubic closest packed structures. When the pseudo-spinel crystal is incontact with the layered rock-salt crystal and the rock-salt crystal,there is a crystal plane at which orientations of cubic closest packedstructures composed of anions are aligned. Note that a space group ofthe layered rock-salt crystal and the pseudo-spinel crystal is R-3m,which is different from a space group Fm-3m of a rock-salt crystal (aspace group of a general rock-salt crystal) and a space group Fd-3m of arock-salt crystal (a space group of a rock-salt crystal having thesimplest symmetry); thus, the Miller index of the crystal planesatisfying the above conditions in the layered rock-salt crystal and thepseudo-spinel crystal is different from that in the rock-salt crystal.In this specification, a state where the orientations of the cubicclosest packed structures composed of anions in the layered rock-saltcrystal, the pseudo-spinel crystal, and the rock-salt crystal arealigned is referred to as a state where crystal orientations aresubstantially aligned in some cases.

Note that in the unit cell of the pseudo-spinel crystal structure,coordinates of cobalt and oxygen can be represented by Co (0, 0, 0.5)and O (0, 0, x) within the range of 0.20≤x≤0.25.

In the positive electrode active material of one embodiment of thepresent invention, a difference between the volume per unit cell with adepth of charge of 0 and the volume per unit cell of the pseudo-spinelcrystal structure with a depth of charge of 0.82 is preferably less thanor equal to 2.5%, further preferably less than or equal to 2.2%.

The pseudo-spinel crystal structure has diffraction peaks at 2θ of19.30±0.20° (greater than or equal to 19.10° and less than or equal to19.50°) and 2θ of 45.55±0.10° (greater than or equal to 45.45° and lessthan or equal to 45.65°). More specifically, sharp diffraction peaksappear at 2θ of 19.30±0.10° (greater than or equal to 19.20° and lessthan or equal to 19.40°) and 2θ of 45.55±0.05° (greater than or equal to45.50° and less than or equal to 45.60°).

Note that although the positive electrode active material of oneembodiment of the present invention has the pseudo-spinel crystalstructure when being charged with high voltage, not all the particlesnecessarily have the pseudo-spinel crystal structure. The particles mayhave another crystal structure, or may partly be amorphous. Note thatwhen the XRD patterns are analyzed by the Rietveld analysis, thepseudo-spinel crystal structure preferably accounts for more than orequal to 50 wt %, further preferably more than or equal to 60 wt %,still further preferably more than or equal to 66 wt % of the positiveelectrode active material. The positive electrode active material inwhich the pseudo-spinel crystal structure accounts for more than orequal to 50 wt %, further preferably more than or equal to 60 wt %,still further preferably more than or equal to 66 wt % can havesufficiently good cycle performance.

The number of atoms of the element X is preferably larger than 0.001times and less than or equal to 0.1 times, further preferably largerthan 0.01 times and less than 0.04 times, still further preferablyapproximately 0.02 times the number of atoms of the element Me. Theconcentration of the element X described here may be a value obtained byelement analysis on the entire particle of the positive electrode activematerial using ICP-MS or the like, or may be a value based on the ratioof the raw materials mixed in the process of forming the positiveelectrode active material, for example.

In the case where cobalt and nickel are included as the element Me, theproportion of nickel atoms (Ni) in the sum of cobalt atoms and nickelatoms (Co+Ni) (Ni/(Co+Ni)) is preferably less than 0.1, furtherpreferably less than or equal to 0.075.

This embodiment can be used in appropriate combination with the otherembodiments.

Embodiment 3

In this embodiment, examples of materials that can be used in asecondary battery containing the positive electrode active material 100described in the above embodiments will be described. In thisembodiment, a secondary battery in which a positive electrode, anegative electrode, and an electrolyte solution are wrapped in anexterior body will be described as an example.

[Positive Electrode]

The positive electrode includes a positive electrode active materiallayer and a positive electrode current collector.

<Positive Electrode Active Material Layer>

The positive electrode active material layer contains at least apositive electrode active material. The positive electrode activematerial layer may contain, in addition to the positive electrode activematerial, other materials such as a coating film of the active materialsurface, a conductive additive, and a binder.

As the positive electrode active material, the positive electrode activematerial 100 described in the above embodiment can be used. A secondarybattery including the positive electrode active material 100 describedin the above embodiment can have high capacity and excellent cycleperformance.

Examples of the conductive additive include a carbon material, a metalmaterial, and a conductive ceramic material. Alternatively, a fibermaterial may be used as the conductive additive. The content of theconductive additive in the active material layer is preferably greaterthan or equal to 1 wt % and less than or equal to 10 wt %, furtherpreferably greater than or equal to 1 wt % and less than or equal to 5wt %.

A network for electric conduction can be formed in the active materiallayer by the conductive additive. The conductive additive also allowsthe maintenance of a path for electric conduction between the positiveelectrode active materials. The addition of the conductive additive tothe active material layer increases the electric conductivity of theactive material layer.

Examples of the conductive additive include natural graphite, artificialgraphite such as mesocarbon microbeads, and carbon fiber. As carbonfiber, mesophase pitch-based carbon fiber and isotropic pitch-basedcarbon fiber can be used. Other examples of carbon fiber include carbonnanofiber and carbon nanotube. Carbon nanotube can be formed by, forexample, a vapor deposition method. Other examples of the conductiveadditive include carbon materials such as carbon black (e.g., acetyleneblack (AB)), graphite (black lead) particles, graphene, and fullerene.Alternatively, metal powder or metal fibers of copper, nickel, aluminum,silver, gold, or the like, a conductive ceramic material, or the likecan be used.

Alternatively, a graphene compound may be used as the conductiveadditive.

A graphene compound has excellent electrical characteristics of highconductivity and excellent physical properties of high flexibility andhigh mechanical strength in some cases. A graphene compound has a planarshape. A graphene compound enables low-resistance surface contact.Furthermore, a graphene compound has extremely high conductivity evenwith a small thickness in some cases and thus allows a conductive pathto be formed in an active material layer efficiently even with a smallamount. Hence, a graphene compound is preferably used as the conductiveadditive, in which case the area where the active material and theconductive additive are in contact with each other can be increased. Agraphene compound that is the conductive additive is preferably formedusing a spray dry apparatus as a coating film to cover the entiresurface of the active material. In addition, a graphene compound ispreferable because electrical resistance can be reduced in some cases.Here, it is particularly preferable to use, for example, graphene,multilayer graphene, or RGO as a graphene compound. Note that RGO refersto a compound obtained by reducing graphene oxide (GO), for example.

In the case where an active material with a small particle size (e.g., 1μm or less) is used, the specific surface area of the active material islarge and thus more conductive paths for the active materials areneeded. Consequently, the amount of conductive additive tends toincrease, and the carried amount of active material tends to decreaserelatively. When the carried amount of active material decreases, thecapacity of the secondary battery also decreases. In such a case, agraphene compound is particularly preferably used as the conductiveadditive because a graphene compound can efficiently form a conductivepath even with a small amount and does not decrease the carried amountof active material.

As a graphene compound, graphene or multilayer graphene may be used, forexample. Here, the graphene compound preferably has a sheet-like shape.The graphene compound may have a sheet-like shape formed of a pluralityof sheets of multilayer graphene and/or a plurality of sheets ofgraphene that partly overlap each other.

In the longitudinal cross section of the active material layer, thesheet-like graphene compounds are preferably dispersed substantiallyuniformly in the active material layer. The plurality of graphenecompounds are preferably formed to partly coat or adhere to the surfacesof a plurality of particles of the positive electrode active material sothat the graphene compounds make surface contact with the particles ofthe positive electrode active material.

Here, the plurality of graphene compounds are bonded to each other,thereby forming a net-like graphene compound sheet (hereinafter referredto as a graphene compound net or a graphene net). The graphene netcovering the active material can function as a binder for bonding activematerials. The amount of binder can thus be reduced, or the binder doesnot have to be used. This can increase the proportion of the activematerial in the electrode volume or the electrode weight. That is, thecapacity of the secondary battery can be increased.

Here, it is preferable to perform reduction after a layer to be theactive material layer is formed in such a manner that graphene oxide isused as the graphene compound and mixed with an active material. Whengraphene oxide with extremely high dispersibility in a polar solvent isused to form the graphene compounds, the graphene compounds can besubstantially uniformly dispersed in the active material layer. Thesolvent is removed by volatilization from a dispersion medium containingthe uniformly dispersed graphene oxide to reduce the graphene oxide;hence, the graphene compounds remaining in the active material layerpartly overlap each other and are dispersed such that surface contact ismade, thereby forming a three-dimensional conduction path. Note thatgraphene oxide can be reduced by heat treatment or with the use of areducing agent, for example.

Unlike a particle of conductive additive such as acetylene black, whichmakes point contact with an active material, the graphene compound iscapable of making low-resistance surface contact; accordingly, theelectrical conduction between the particles of the positive electrodeactive material and the graphene compound can be improved with a smalleramount of the graphene compound than that of a normal conductiveadditive. This can increase the proportion of the positive electrodeactive material in the active material layer. Thus, the dischargecapacity of the secondary battery can be increased.

With a spray dry apparatus, a graphene compound serving as a conductiveadditive can be formed in advance as a coating film to cover the entiresurface of the active material, and a conductive path can be formedbetween the active materials using the graphene compound.

As the binder, a rubber material such as styrene-butadiene rubber (SBR),styrene-isoprene-styrene rubber, acrylonitrile-butadiene rubber,butadiene rubber, or ethylene-propylene-diene copolymer can be used, forexample. Alternatively, fluororubber can be used as the binder.

For the binder, for example, water-soluble polymers are preferably used.As the water-soluble polymers, for example, a polysaccharide can beused. As the polysaccharide, for example, a cellulose derivative such ascarboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, and regenerated celluloseor starch can be used. It is further preferred that such water-solublepolymers be used in combination with any of the above rubber materials.

Alternatively, as the binder, a material such as polystyrene,poly(methyl acrylate), poly(methyl methacrylate) (polymethylmethacrylate, PMMA), sodium polyacrylate, polyvinyl alcohol (PVA),polyethylene oxide (PEO), polypropylene oxide, polyimide, polyvinylchloride, polytetrafluoroethylene, polyethylene, polypropylene,polyisobutylene, polyethylene terephthalate, nylon, polyvinylidenefluoride (PVDF), polyacrylonitrile (PAN), ethylene-propylene-dienepolymer, polyvinyl acetate, or nitrocellulose is preferably used.

A plurality of the above materials may be used in combination as thebinder.

For example, a material having a significant viscosity modifying effectand another material may be used in combination. For example, a rubbermaterial or the like has high adhesion or high elasticity but may havedifficulty in viscosity modification when mixed in a solvent. In such acase, a rubber material or the like is preferably mixed with a materialhaving a significant viscosity modifying effect, for example. As amaterial having a significant viscosity modifying effect, for example, awater-soluble polymer is preferably used. An example of a water-solublepolymer having a significant viscosity modifying effect is theabove-mentioned polysaccharide; for example, a cellulose derivative suchas carboxymethyl cellulose (CMC), methyl cellulose, ethyl cellulose,hydroxypropyl cellulose, diacetyl cellulose, or regenerated cellulose,or starch can be used.

Note that a cellulose derivative such as carboxymethyl cellulose obtainsa higher solubility when converted into a salt such as a sodium salt oran ammonium salt of carboxymethyl cellulose, and accordingly, easilyexerts an effect as a viscosity modifier. The high solubility can alsoincrease the dispersibility of an active material and other componentsin the formation of slurry for an electrode. In this specification,cellulose and a cellulose derivative used as a binder of an electrodeinclude salts thereof.

The water-soluble polymers stabilize viscosity by being dissolved inwater and allow stable dispersion of the active material and anothermaterial combined as a binder, such as styrene-butadiene rubber, in anaqueous solution. Furthermore, a water-soluble polymer is expected to beeasily and stably adsorbed on the active material surface because it hasa functional group. Many cellulose derivatives, such as carboxymethylcellulose, have functional groups such as a hydroxyl group and acarboxyl group. Because of functional groups, polymers are expected tointeract with each other and cover the active material surface in alarge area.

In the case where the binder covering or being in contact with theactive material surface forms a film, the film is expected to serve as apassivation film to suppress the decomposition of the electrolytesolution. Here, the passivation film refers to a film without electronicconductivity or a film with extremely low electric conductivity, and cansuppress the decomposition of an electrolyte solution at a potential atwhich a battery reaction occurs in the case where the passivation filmis formed on the active material surface, for example. It is preferredthat the passivation film can conduct lithium ions while suppressingelectric conduction.

<Positive Electrode Current Collector>

The positive electrode current collector can be formed using a materialthat has high conductivity, which is for example a metal such asstainless steel, gold, platinum, aluminum, or titanium or an alloythereof. It is preferred that a material used for the positive electrodecurrent collector not dissolve at the potential of the positiveelectrode. Alternatively, it is possible to use an aluminum alloy towhich an element that improves heat resistance, such as silicon,titanium, neodymium, scandium, or molybdenum, is added. Stillalternatively, the positive electrode current collector may be formedusing a metal element that forms silicide by reacting with silicon.Examples of the metal element that forms silicide by reacting withsilicon include zirconium, titanium, hafnium, vanadium, niobium,tantalum, chromium, molybdenum, tungsten, cobalt, and nickel. Thecurrent collector can have any of various shapes including a foil-likeshape, a plate-like shape (sheet-like shape), a net-like shape, apunching-metal shape, and an expanded-metal shape. The current collectorpreferably has a thickness of greater than or equal to 5 μm and lessthan or equal to 30 μm.

[Negative Electrode]

The negative electrode includes a negative electrode active materiallayer and a negative electrode current collector. The negative electrodeactive material layer may contain a conductive additive and a binder.

<Negative Electrode Active Material>

As a negative electrode active material, for example, an alloy-basedmaterial or a carbon-based material can be used.

For the negative electrode active material, an element that enablescharge-discharge reactions by alloying and dealloying reactions withlithium can be used. For example, a material containing at least one ofsilicon, tin, gallium, aluminum, germanium, lead, antimony, bismuth,silver, zinc, cadmium, indium, and the like can be used. Such elementshave higher capacity than carbon, and silicon in particular has a hightheoretical capacity of 4200 mAh/g. For this reason, silicon ispreferably used as the negative electrode active material.Alternatively, a compound containing any of the above elements may beused. Examples of the compound include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂,Mg₂Sn, SnS₂, V₂Sn₃, FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb,CeSb₃, LaSn₃, La₃Co₂Sn₇, CoSb₃, InSb, and SbSn. Here, an element thatenables charge-discharge reactions by alloying and dealloying reactionswith lithium and a compound containing the element, for example, may bereferred to as an alloy-based material.

In this specification and the like, SiO refers, for example, to siliconmonoxide. Note that SiO can alternatively be expressed as SiO_(x). Here,x preferably has an approximate value of 1. For example, x is preferablymore than or equal to 0.2 and less than or equal to 1.5, furtherpreferably more than or equal to 0.3 and less than or equal to 1.2.

As the carbon-based material, graphite, graphitizing carbon (softcarbon), non-graphitizing carbon (hard carbon), carbon nanotube,graphene, carbon black, and the like can be used.

Examples of graphite include artificial graphite and natural graphite.Examples of artificial graphite include meso-carbon microbeads (MCMB),coke-based artificial graphite, and pitch-based artificial graphite. Asartificial graphite, spherical graphite having a spherical shape can beused. For example, MCMB is preferably used because it may have aspherical shape. Moreover, MCMB may preferably be used because it isrelatively easy to have a small surface area. Examples of naturalgraphite include flake graphite and spherical natural graphite.

Graphite has a low potential substantially equal to that of a lithiummetal (higher than or equal to 0.05 V and lower than or equal to 0.3 Vvs. Li/Li⁺) when lithium ions are intercalated into graphite (when alithium-graphite intercalation compound is formed). For this reason, alithium-ion secondary battery can have a high operating voltage. Inaddition, graphite is preferred because of its advantages such as arelatively high capacity per unit volume, relatively small volumeexpansion, low cost, and higher level of safety than that of a lithiummetal.

Alternatively, for the negative electrode active material, oxide such astitanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the negative electrode active material,Li_(3-x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a positive electrode active material that doesnot contain lithium ions, such as V₂O₅ or Cr₃O₈. Note that in the caseof using a material containing lithium ions as a positive electrodeactive material, the nitride containing lithium and a transition metalcan be used for the negative electrode active material by extracting thelithium ions contained in the positive electrode active material inadvance.

Alternatively, a material that causes a conversion reaction can be usedfor the negative electrode active material. For example, a transitionmetal oxide that does not form an alloy with lithium, such as cobaltoxide (CoO), nickel oxide (NiO), and iron oxide (FeO), may be used.Other examples of the material that causes a conversion reaction includeoxides such as Fe₂O₃, CuO, Cu₂O, RuO₂, and Cr₂O₃, sulfides such asCoS_(0.89), NiS, and CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃.

For the conductive additive and the binder that can be included in thenegative electrode active material layer, materials similar to those ofthe conductive additive and the binder that can be included in thepositive electrode active material layer can be used.

<Negative Electrode Current Collector>

For the negative electrode current collector, a material similar to thatof the positive electrode current collector can be used. Note that amaterial that is not alloyed with carrier ions, such as lithium, ispreferably used for the negative electrode current collector.

[Electrolyte Solution]

The electrolyte solution contains a solvent and an electrolyte. As thesolvent of the electrolyte solution, an aprotic organic solvent ispreferably used. For example, one of ethylene carbonate (EC), propylenecarbonate (PC), butylene carbonate, chloroethylene carbonate, vinylenecarbonate, γ-butyrolactone, γ-valerolactone, dimethyl carbonate (DMC),diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl formate,methyl acetate, ethyl acetate, methyl propionate, ethyl propionate,propyl propionate, methyl butyrate, 1,3-dioxane, 1,4-dioxane,dimethoxyethane (DME), dimethyl sulfoxide, diethyl ether, methyldiglyme, acetonitrile, benzonitrile, tetrahydrofuran, sulfolane, andsultone can be used, or two or more of these solvents can be used in anappropriate combination in an appropriate ratio.

Alternatively, the use of one or more ionic liquids (room temperaturemolten salts) that are less likely to burn and volatize as the solventof the electrolyte solution can prevent a secondary battery fromexploding or catching fire even when the secondary battery internallyshorts out or the internal temperature increases owing to overcharge orthe like. An ionic liquid contains a cation and an anion, specifically,an organic cation and an anion. Examples of the organic cation used forthe electrolyte solution include aliphatic onium cations such as aquaternary ammonium cation, a tertiary sulfonium cation, and aquaternary phosphonium cation, and aromatic cations such as animidazolium cation and a pyridinium cation. Examples of the anion usedfor the electrolyte solution include a monovalent amide-based anion, amonovalent methide-based anion, a fluorosulfonate anion, aperfluoroalkylsulfonate anion, a tetrafluoroborate anion, aperfluoroalkylborate anion, a hexafluorophosphate anion, and aperfluoroalkylphosphate anion.

As an electrolyte dissolved in the above-described solvent, one oflithium salts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiAlCl₄, LiSCN,LiBr, LiI, Li₂SO₄, Li₂B₁₀Cl₁₀, Li₂Bi₂Cl₁₂, LiCF₃SO₃, LiC₄F₉SO₃,LiC(CF₃SO₂)₃, LiC(C₂F₅SO₂)₃, LiN(CF₃SO₂)₂, LiN(C₄F₉SO₂)(CF₃SO₂), andLiN(C₂F₅SO₂)₂ can be used, or two or more of these lithium salts can beused in an appropriate combination in an appropriate ratio.

The electrolyte solution used for a secondary battery is preferablyhighly purified and contains small numbers of dust particles andelements other than the constituent elements of the electrolyte solution(hereinafter also simply referred to as “impurities”). Specifically, theweight ratio of impurities to the electrolyte solution is preferablyless than or equal to 1%, further preferably less than or equal to 0.1%,still further preferably less than or equal to 0.01%.

An additive agent such as vinylene carbonate, propane sultone (PS),tert-butylbenzene (TBB), fluoroethylene carbonate (FEC), lithiumbis(oxalate)borate (LiBOB), or a dinitrile compound such assuccinonitrile or adiponitrile may be added to the electrolyte solution.The concentration of the material to be added in the whole solvent is,for example, higher than or equal to 0.1 wt % and lower than or equal to5 wt %.

Alternatively, a polymer gelled electrolyte obtained in such a mannerthat a polymer is swelled with an electrolyte solution may be used.

When a polymer gel electrolyte is used, safety against liquid leakageand the like is improved. Furthermore, a secondary battery can bethinner and more lightweight.

As a polymer that undergoes gelation, a silicone gel, an acrylic gel, anacrylonitrile gel, a polyethylene oxide-based gel, a polypropyleneoxide-based gel, a fluorine-based polymer gel, or the like can be used.

Examples of the polymer include a polymer having a polyalkylene oxidestructure, such as polyethylene oxide (PEO); PVDF; polyacrylonitrile;and a copolymer containing any of them. For example, PVDF-HFP, which isa copolymer of PVDF and hexafluoropropylene (HFP), can be used. Theformed polymer may be porous.

Instead of the electrolyte solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including ahigh-molecular material such as a PEO (polyethylene oxide)-basedhigh-molecular material may alternatively be used. When the solidelectrolyte is used, a separator and a spacer are not necessary.Furthermore, the battery can be entirely solidified; therefore, there isno possibility of liquid leakage and thus the safety of the battery isdramatically increased.

Examples of the sulfide-based solid electrolyte include athio-silicon-based material (e.g., Li₁₀GeP₂S₁₂ andLi_(3.25)Ge_(0.25)P_(0.75)S₄), sulfide glass (e.g., 70Li₂S.30P₂S₅,30Li₂S.26B₂S₃.44LiI, 63Li₂S.38SiS₂.1Li₃PO₄, 57Li₂S.38SiS₂.5Li₄SiO₄, and50Li₂S.50GeS₂), and sulfide-based crystallized glass (e.g., Li₇P₃S₁₁ andLi_(3.25)P_(0.95)S₄). The sulfide-based solid electrolyte has advantagessuch as high conductivity of some materials, low-temperature synthesis,and ease of maintaining a conduction path after charge and dischargebecause of its relative softness.

Examples of the oxide-based solid electrolyte include a material with aperovskite crystal structure (e.g., La_(2/3−x)Li_(3x)TiO₃), a materialwith a NASICON crystal structure (e.g., Li_(1−x)Al_(x)Ti_(2−x)(PO₄)₃), amaterial with a garnet crystal structure (e.g., Li₇La₃Zr₂O₁₂), amaterial with a LISICON crystal structure (e.g., Li₁₄ZnGe₄O₁₆), LLZO(Li₇La₃Zr₂O₁₂), oxide glass (e.g., Li₃PO₄—Li₄SiO₄ and50Li₄SiO₄.50Li₃BO₃), and oxide-based crystallized glass (e.g.,Li_(1.07)Al_(0.69)Ti_(1.46)(PO₄)₃ and Li_(1.5)Al_(0.5)Ge_(1.5)(PO₄)₃).The oxide-based solid electrolyte has an advantage of stability in theair.

Examples of the halide-based solid electrolyte include LiAlCl₄,Li₃InBr₆, LiF, LiCl, LiBr, and LiI. Moreover, a composite material inwhich pores of porous aluminum oxide or porous silica are filled withsuch a halide-based solid electrolyte can be used as the solidelectrolyte.

Alternatively, different solid electrolytes may be mixed and used.

In particular, Li_(1+x)Al_(x)Ti_(2−x)(PO₄)₃ (0<x<1) with a NASICONcrystal structure (hereinafter LATP) is preferable because LATP containsaluminum and titanium, each of which is the element the positiveelectrode active material used for the secondary battery of oneembodiment of the present invention is allowed to contain, and thus asynergistic effect of improving the cycle performance is expected.Moreover, higher productivity due to the reduction in the number ofsteps is expected. Note that in this specification and the like, amaterial with a NASICON crystal structure refers to a compound that isrepresented by M₂(XO₄)₃ (M: transition metal; X: S, P, As, Mo, W, or thelike) and has a structure in which MO₆ octahedra and XO₄ tetrahedra thatshare common corners are arranged three-dimensionally.

[Separator]

The secondary battery preferably includes a separator. As the separator,for example, paper; nonwoven fabric; glass fiber; ceramics; or syntheticfiber containing nylon (polyamide), vinylon (polyvinyl alcohol-basedfiber), polyester, acrylic, polyolefin, or polyurethane can be used. Theseparator is preferably formed to have an envelope-like shape and placedto wrap one of the positive electrode and the negative electrode.

The separator may have a multilayer structure. For example, an organicmaterial film of polypropylene, polyethylene, or the like can be coatedwith a ceramic-based material, a fluorine-based material, apolyamide-based material, a mixture thereof, or the like. Examples ofthe ceramic-based material include aluminum oxide particles and siliconoxide particles. Examples of the fluorine-based material include PVDFand polytetrafluoroethylene. Examples of the polyamide-based materialinclude nylon and aramid (meta-based aramid and para-based aramid).

When the separator is coated with the ceramic-based material, theoxidation resistance is improved; hence, deterioration of the separatorin charge and discharge at high voltage can be suppressed and thus thereliability of the secondary battery can be improved. In addition, whenthe separator is coated with the fluorine-based material, the separatoris easily brought into close contact with an electrode, resulting inhigh output characteristics. When the separator is coated with thepolyamide-based material, in particular, aramid, heat resistance isimproved; thus, the safety of the secondary battery is improved.

For example, both surfaces of a polypropylene film may be coated with amixed material of aluminum oxide and aramid. Alternatively, a surface ofthe polypropylene film that is in contact with the positive electrodemay be coated with the mixed material of aluminum oxide and aramid, anda surface of the polypropylene film that is in contact with the negativeelectrode may be coated with the fluorine-based material.

With the use of a separator having a multilayer structure, the capacityper volume of the secondary battery can be increased because the safetyof the secondary battery can be maintained even when the total thicknessof the separator is small.

[Exterior Body]

For an exterior body included in the secondary battery, a metal materialsuch as aluminum or a resin material can be used, for example. Anexterior body in the form of a film can also be used. As the film, forexample, it is possible to use a film having a three-layer structure inwhich a highly flexible metal thin film of aluminum, stainless steel,copper, nickel, or the like is provided over a film formed of a materialsuch as polyethylene, polypropylene, polycarbonate, ionomer, orpolyamide, and an insulating synthetic resin film of a polyamide-basedresin, a polyester-based resin, or the like is provided as the outersurface of the exterior body over the metal thin film.

Embodiment 4

In this embodiment, examples of a shape of a secondary battery includingthe positive electrode active material 100 described in the aboveembodiment are described. For the materials used for the secondarybattery described in this embodiment, the description of the aboveembodiment can be referred to.

[Coin-Type Secondary Battery]

First, an example of a coin-type secondary battery is described. FIG. 5Ais an external view of a coin-type (single-layer flat type) secondarybattery, and FIG. 5B is a cross-sectional view thereof.

In a coin-type secondary battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308.

Note that only one surface of each of the positive electrode 304 and thenegative electrode 307 used for the coin-type secondary battery 300 isprovided with an active material layer.

For the positive electrode can 301 and the negative electrode can 302, ametal having corrosion resistance to an electrolyte solution, such asnickel, aluminum, or titanium, an alloy of such a metal, or an alloy ofsuch a metal and another metal (e.g., stainless steel) can be used. Thepositive electrode can 301 and the negative electrode can 302 arepreferably covered with nickel, aluminum, or the like in order toprevent corrosion due to the electrolyte solution. The positiveelectrode can 301 and the negative electrode can 302 are electricallyconnected to the positive electrode 304 and the negative electrode 307,respectively.

The coin-type secondary battery 300 is manufactured in the followingmanner: the negative electrode 307, the positive electrode 304, and aseparator 310 are immersed in the electrolyte solution; as illustratedin FIG. 5B, the positive electrode 304, the separator 310, the negativeelectrode 307, and the negative electrode can 302 are stacked in thisorder with the positive electrode can 301 positioned at the bottom; andthen the positive electrode can 301 and the negative electrode can 302are subjected to pressure bonding with the gasket 303 locatedtherebetween.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 304, the coin-typesecondary battery 300 with high capacity and excellent cycle performancecan be obtained.

Here, a current flow in charging the secondary battery is described withreference to FIG. 5C. When a secondary battery using lithium is regardedas a closed circuit, lithium ions transfer and a current 78 i flows inthe same direction. Note that in the secondary battery using lithium, ananode and a cathode change places in charging and discharging, and anoxidation reaction and a reduction reaction occur on the correspondingsides; hence, an electrode with a high reaction potential is called apositive electrode and an electrode with a low reaction potential iscalled a negative electrode. For this reason, in this specification, thepositive electrode is referred to as a “positive electrode” or a “pluselectrode” and the negative electrode is referred to as a “negativeelectrode” or a “minus electrode” in all the cases where charging isperformed, discharging is performed, a reverse pulse current issupplied, and a charge current is supplied. The use of terms an “anode”and a “cathode” related to oxidation reaction and reduction reactionmight cause confusion because the anode and the cathode interchange incharging and in discharging. Thus, the terms “anode” and “cathode” arenot used in this specification. If the term the “anode” or the “cathode”is used, it should be clearly mentioned that the anode or the cathode iswhich of the one in charging or in discharging and corresponds to whichof the positive electrode (plus electrode) or the negative electrode(minus electrode).

Two terminals illustrated in FIG. 5C are connected to a charger, and thesecondary battery 300 is charged. As the charging of the secondarybattery 300 proceeds, a potential difference between electrodesincreases.

[Cylindrical Secondary Battery]

Next, an example of a cylindrical secondary battery is described withreference to FIG. 6. FIG. 6A illustrates an external view of acylindrical secondary battery 600. FIG. 6B is a diagram schematicallyillustrating a cross section of the cylindrical secondary battery 600.As illustrated in FIG. 6B, the cylindrical secondary battery 600includes a positive electrode cap (battery lid) 601 on a top surface anda battery can (outer can) 602 on a side surface and a bottom surface.The positive electrode cap and the battery can (outer can) 602 areinsulated from each other by a gasket (insulating packing) 610.

Inside the battery can 602 having a hollow cylindrical shape, a batteryelement in which a belt-like positive electrode 604 and a belt-likenegative electrode 606 are wound with a separator 605 locatedtherebetween is provided. Although not illustrated, the battery elementis wound centering around a center pin. One end of the battery can 602is closed and the other end thereof is open. For the battery can 602, ametal having a corrosion-resistant property to an electrolyte solution,such as nickel, aluminum, or titanium, an alloy of such a metal, or analloy of such a metal and another metal (e.g., stainless steel) can beused. Alternatively, the battery can 602 is preferably covered withnickel, aluminum, or the like in order to prevent corrosion due to theelectrolyte solution. Inside the battery can 602, the battery element inwhich the positive electrode, the negative electrode, and the separatorare wound is sandwiched between a pair of insulating plates 608 and 609that face each other. Furthermore, a nonaqueous electrolyte (notillustrated) is injected inside the battery can 602 provided with thebattery element. As the nonaqueous electrolyte, a nonaqueous electrolytethat is similar to that for the coin-type secondary battery can be used.

Since a positive electrode and a negative electrode that are used for acylindrical storage battery are wound, active materials are preferablyformed on both surfaces of a current collector. A positive electrodeterminal (positive electrode current collector lead) 603 is connected tothe positive electrode 604, and a negative electrode terminal (negativeelectrode current collector lead) 607 is connected to the negativeelectrode 606. For both the positive electrode terminal 603 and thenegative electrode terminal 607, a metal material such as aluminum canbe used. The positive electrode terminal 603 and the negative electrodeterminal 607 are resistance-welded to a safety valve mechanism 612 andthe bottom of the battery can 602, respectively. The safety valvemechanism 612 is electrically connected to the positive electrode cap601 through a PTC element (Positive Temperature Coefficient) 611. Thesafety valve mechanism 612 cuts off electrical connection between thepositive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. In addition, the PTC element 611 is a thermally sensitiveresistor whose resistance increases as temperature rises, and limits theamount of current by increasing the resistance to prevent abnormal heatgeneration. Barium titanate (BaTiO₃)-based semiconductor ceramics or thelike can be used for the PTC element.

Furthermore, as illustrated in FIG. 6C, a plurality of secondarybatteries 600 may be sandwiched between a conductive plate 613 and aconductive plate 614 to form a module 615. The plurality of secondarybatteries 600 may be connected in parallel, connected in series, orconnected in series after being connected in parallel. By forming themodule 615 including the plurality of secondary batteries 600, largepower can be extracted.

FIG. 6D is a top view of the module 615. The conductive plate 613 isshown by a dotted line for clarity of the drawing. As illustrated inFIG. 6D, the module 615 may include a wiring 616 which electricallyconnects the plurality of secondary batteries 600 to each other. It ispossible to provide the conductive plate over the wiring 616 to overlapwith each other. In addition, a temperature control device 617 may beprovided between the plurality of secondary batteries 600. The secondarybatteries 600 can be cooled with the temperature control device 617 whenoverheated, whereas the secondary batteries 600 can be heated with thetemperature control device 617 when cooled too much. Thus, theperformance of the module 615 is less likely to be influenced by theoutside temperature. A heating medium included in the temperaturecontrol device 617 preferably has an insulating property andincombustibility.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 604, the cylindricalsecondary battery 600 with high capacity and excellent cycle performancecan be obtained.

[Structure Examples of Secondary Battery]

Other structure examples of secondary batteries are described using FIG.7 to FIG. 11.

FIG. 7A and FIG. 7B are external views of a secondary battery. Asecondary battery 913 is connected to an antenna 914 and an antenna 915through a circuit board 900. A label 910 is attached to the secondarybattery 913. Moreover, as illustrated in FIG. 7B, the secondary battery913 is connected to a terminal 951 and a terminal 952.

The circuit board 900 includes a terminal 911 and a circuit 912. Theterminal 911 is connected to the terminal 951, the terminal 952, theantenna 914, the antenna 915, and the circuit 912. Note that a pluralityof terminals 911 serving as a control signal input terminal, a powersupply terminal, and the like may be provided.

The circuit 912 may be provided on the rear surface of the circuit board900. Note that the shape of the antenna 914 and the antenna 915 is notlimited to a coil shape and may be a linear shape or a plate shape. Anantenna such as a planar antenna, an aperture antenna, a traveling-waveantenna, an EH antenna, a magnetic-field antenna, or a dielectricantenna may be used. Alternatively, the antenna 914 or the antenna 915may be a flat-plate conductor. This flat-plate conductor can serve asone of conductors for electric field coupling. That is, the antenna 914or the antenna 915 can serve as one of two conductors of a capacitor.Thus, electric power can be transmitted and received not only by anelectromagnetic field or a magnetic field but also by an electric field.

The line width of the antenna 914 is preferably larger than the linewidth of the antenna 915. This makes it possible to increase the amountof power received by the antenna 914.

The secondary battery includes a layer 916 between the antennas 914 and915 and the secondary battery 913. The layer 916 has a function ofblocking an electromagnetic field from the secondary battery 913, forexample. As the layer 916, for example, a magnetic body can be used.

Note that the structure of the secondary battery is not limited to thatillustrated in FIG. 7.

For example, as illustrated in FIG. 8A and FIG. 8B, an antenna may beprovided for each of a pair of opposite surfaces of the secondarybattery 913 illustrated in FIG. 7A and FIG. 7B. FIG. 8A is an externalview illustrating one of the pair of surfaces, and FIG. 8B is anexternal view illustrating the other of the pair of surfaces. Note thatfor portions similar to those in FIG. 7A and FIG. 7B, the description ofthe secondary battery illustrated in FIG. 7A and FIG. 7B can beappropriately referred to.

As illustrated in FIG. 8A, the antenna 914 is provided on one of thepair of surfaces of the secondary battery 913 with the layer 916 locatedtherebetween, and as illustrated in FIG. 8B, an antenna 918 is providedon the other of the pair of surfaces of the secondary battery 913 with alayer 917 located therebetween. The layer 917 has a function of blockingan electromagnetic field by the secondary battery 913, for example. Asthe layer 917, for example, a magnetic body can be used.

With the above structure, both of the antenna 914 and the antenna 918can be increased in size. The antenna 918 has a function ofcommunicating data with an external device, for example. An antenna witha shape that can be applied to the antenna 914, for example, can be usedas the antenna 918. As a system for communication using the antenna 918between the secondary battery and another device, a response method thatcan be used between the secondary battery and another device, such asnear field communication (NFC), can be employed.

Alternatively, as illustrated in FIG. 8C, the secondary battery 913illustrated in FIG. 7A and FIG. 7B may be provided with a display device920. The display device 920 is electrically connected to the terminal911. Note that the label 910 is not necessarily provided in a portionwhere the display device 920 is provided. Note that for portions similarto those in FIG. 7A and FIG. 7B, the description of the secondarybattery illustrated in FIG. 7A and FIG. 7B can be appropriately referredto.

The display device 920 can display, for example, an image showingwhether or not charging is being carried out, an image showing theamount of stored power, or the like. As the display device 920,electronic paper, a liquid crystal display device, an electroluminescent(EL) display device, or the like can be used. For example, the use ofelectronic paper can reduce power consumption of the display device 920.

Alternatively, as illustrated in FIG. 8D, the secondary battery 913illustrated in FIG. 7A and FIG. 7B may be provided with a sensor 921.The sensor 921 is electrically connected to the terminal 911 via aterminal 922. Note that for portions similar to those in FIG. 7A andFIG. 7B, the description of the secondary battery illustrated in FIG. 7Aand FIG. 7B can be appropriately referred to.

The sensor 921 has a function of measuring, for example, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredrays. With provision of the sensor 921, for example, data on anenvironment where the secondary battery is placed (e.g., temperature orthe like) can be detected and stored in a memory inside the circuit 912.

Furthermore, structure examples of the secondary battery 913 aredescribed using FIG. 9 and FIG. 10.

The secondary battery 913 illustrated in FIG. 9A includes a wound body950 provided with the terminal 951 and the terminal 952 inside a housing930. The wound body 950 is immersed in an electrolyte solution insidethe housing 930. The terminal 952 is in contact with the housing 930,and the use of an insulator or the like inhibits contact between theterminal 951 and the housing 930. Note that for convenience, FIG. 9Aillustrates the housing 930 divided into pieces; however, in reality,the wound body 950 is covered with the housing 930 and the terminal 951and the terminal 952 extend to the outside of the housing 930. For thehousing 930, a metal material (e.g., aluminum) or a resin material canbe used.

Note that as illustrated in FIG. 9B, the housing 930 illustrated in FIG.9A may be formed using a plurality of materials. For example, in thesecondary battery 913 illustrated in FIG. 9B, a housing 930 a and ahousing 930 b are bonded to each other, and the wound body 950 isprovided in a region surrounded by the housing 930 a and the housing 930b.

For the housing 930 a, an insulating material such as an organic resincan be used. In particular, when a material such as an organic resin isused for the side on which an antenna is formed, blocking of an electricfield from the secondary battery 913 can be inhibited. When an electricfield is not significantly blocked by the housing 930 a, an antenna suchas the antenna 914 or the antenna 915 may be provided inside the housing930 a. For the housing 930 b, a metal material can be used, for example.

In addition, FIG. 10 illustrates the structure of the wound body 950.The wound body 950 includes a negative electrode 931, a positiveelectrode 932, and separators 933. The wound body 950 is obtained bywinding a sheet of a stack in which the negative electrode 931 overlapswith the positive electrode 932 with the separator 933 sandwichedtherebetween. Note that a plurality of stacks each including thenegative electrode 931, the positive electrode 932, and the separator933 may be stacked.

The negative electrode 931 is connected to the terminal 911 illustratedin FIG. 7 via one of the terminal 951 and the terminal 952. The positiveelectrode 932 is connected to the terminal 911 illustrated in FIG. 7 viathe other of the terminal 951 and the terminal 952.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 932, the secondary battery913 with high capacity and excellent cycle performance can be obtained.

[Laminated Secondary Battery]

Next, examples of a laminated secondary battery are described withreference to FIG. 11 to FIG. 17. When the laminated secondary batteryhas flexibility and is incorporated in an electronic device that has aflexible portion at least partly, the secondary battery can be bentalong the deformed electronic device.

A laminated secondary battery 980 is described using FIG. 11. Thelaminated secondary battery 980 includes a wound body 993 illustrated inFIG. 11A. The wound body 993 includes a negative electrode 994, apositive electrode 995, and separators 996. Like the wound body 950illustrated in FIG. 10, the wound body 993 is obtained by winding asheet of a stack in which the negative electrode 994 overlaps with thepositive electrode 995 with the separator 996 sandwiched therebetween.

Note that the number of stacked layers including the negative electrode994, the positive electrode 995, and the separator 996 may be designedas appropriate depending on required capacity and element volume. Thenegative electrode 994 is connected to a negative electrode currentcollector (not illustrated) via one of a lead electrode 997 and a leadelectrode 998, and the positive electrode 995 is connected to a positiveelectrode current collector (not illustrated) via the other of the leadelectrode 997 and the lead electrode 998.

As illustrated in FIG. 11B, the wound body 993 is packed in a spaceformed by bonding a film 981 and a film 982 having a depressed portionthat serve as exterior bodies by thermocompression bonding or the like,whereby the secondary battery 980 illustrated in FIG. 11C can befabricated. The wound body 993 includes the lead electrode 997 and thelead electrode 998, and is immersed in an electrolyte solution inside aspace surrounded by the film 981 and the film 982 having a depressedportion.

For the film 981 and the film 982 having a depressed portion, a metalmaterial such as aluminum or a resin material can be used, for example.With the use of a resin material for the film 981 and the film 982having a depressed portion, the film 981 and the film 982 having adepressed portion can be changed in their forms when external force isapplied; thus, a flexible storage battery can be fabricated.

Furthermore, although FIG. 11B and FIG. 11C illustrate an example ofusing two films, a space may be formed by bending one film and the woundbody 993 may be packed in the space.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 995, the secondary battery980 with high capacity and excellent cycle performance can be obtained.

Furthermore, FIG. 11 illustrates an example in which the secondarybattery 980 includes a wound body in a space formed by films serving asexterior bodies; however, as illustrated in FIG. 12, for example, asecondary battery may include a plurality of strip-shaped positiveelectrodes, separators, and negative electrodes in a space formed byfilms serving as exterior bodies.

A laminated secondary battery 500 illustrated in FIG. 12A includes apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolyte solution 508, and an exterior body 509. The separator 507 isprovided between the positive electrode 503 and the negative electrode506 that are provided in the exterior body 509. The exterior body 509 isfilled with the electrolyte solution 508. The electrolyte solutiondescribed in Embodiment 2 can be used as the electrolyte solution 508.

In the laminated secondary battery 500 illustrated in FIG. 12A, thepositive electrode current collector 501 and the negative electrodecurrent collector 504 also serve as terminals for electrical contactwith the outside. For this reason, the positive electrode currentcollector 501 and the negative electrode current collector 504 may bearranged so that parts of the positive electrode current collector 501and the negative electrode current collector 504 are exposed to theoutside of the exterior body 509. Alternatively, a lead electrode andthe positive electrode current collector 501 or the negative electrodecurrent collector 504 may be bonded to each other by ultrasonic welding,and instead of the positive electrode current collector 501 and thenegative electrode current collector 504, the lead electrode may beexposed to the outside of the exterior body 509.

As the exterior body 509 of the laminated secondary battery 500, forexample, a laminate film having a three-layer structure can be employedin which a highly flexible metal thin film of aluminum, stainless steel,copper, nickel, or the like is provided over a film formed of a materialsuch as polyethylene, polypropylene, polycarbonate, ionomer, orpolyamide, and an insulating synthetic resin film of a polyamide-basedresin, a polyester-based resin, or the like is provided over the metalthin film as the outer surface of the exterior body.

Furthermore, FIG. 12B illustrates an example of a cross-sectionalstructure of the laminated secondary battery 500. FIG. 12A illustratesan example in which only two current collectors are included forsimplicity, but actually, a plurality of electrode layers are includedas illustrated in FIG. 12B.

In FIG. 12B, the number of electrode layers is 16, for example. Notethat the laminated secondary battery 500 has flexibility even thoughincluding 16 electrode layers. FIG. 12B illustrates a structureincluding 8 layers of negative electrode current collectors 504 and 8layers of positive electrode current collectors 501, i.e., 16 layers intotal. Note that FIG. 12B illustrates a cross section of the leadportion of the negative electrode, and the 8 layers of the negativeelectrode current collectors 504 are bonded to each other by ultrasonicwelding. It is needless to say that the number of electrode layers isnot limited to 16, and may be more than 16 or less than 16. In the caseof a large number of electrode layers, the secondary battery can havehigher capacity. In the case of a small number of electrode layers, thesecondary battery can have smaller thickness and high flexibility.

FIG. 13 and FIG. 14 each illustrate an example of the external view ofthe laminated secondary battery 500. In FIG. 13 and FIG. 14, thelaminated secondary battery 500 includes the positive electrode 503, thenegative electrode 506, the separator 507, the exterior body 509, apositive electrode lead electrode 510, and a negative electrode leadelectrode 511.

FIG. 15A illustrates external views of the positive electrode 503 andthe negative electrode 506. The positive electrode 503 includes thepositive electrode current collector 501, and the positive electrodeactive material layer 502 is formed on a surface of the positiveelectrode current collector 501. The positive electrode 503 alsoincludes a region where the positive electrode current collector 501 ispartly exposed (hereinafter, referred to as a tab region). The negativeelectrode 506 includes the negative electrode current collector 504, andthe negative electrode active material layer 505 is formed on a surfaceof the negative electrode current collector 504. The negative electrode506 also includes a region where the negative electrode currentcollector 504 is partly exposed, that is, a tab region. The areas andthe shapes of the tab regions included in the positive electrode and thenegative electrode are not limited to the examples illustrated in FIG.15A.

[Method for Manufacturing Laminated Secondary Battery]

Here, an example of a method for manufacturing the laminated secondarybattery whose external view is illustrated in FIG. 13 is described usingFIG. 15B and FIG. 15C.

First, the negative electrode 506, the separator 507, and the positiveelectrode 503 are stacked. FIG. 15B illustrates the stack of thenegative electrode 506, the separator 507, and the positive electrode503. An example of using five sets of negative electrodes and four setsof positive electrodes is described here. Next, the tab regions of thepositive electrodes 503 are bonded to each other, and the tab region ofthe positive electrode on the outermost surface and the positiveelectrode lead electrode 510 are bonded to each other. For the bonding,ultrasonic welding can be used, for example. In a similar manner, thetab regions of the negative electrodes 506 are bonded to each other, andthe tab region of the negative electrode on the outermost surface andthe negative electrode lead electrode 511 are bonded to each other.

After that, the negative electrode 506, the separator 507, and thepositive electrode 503 are placed over the exterior body 509.

Subsequently, the exterior body 509 is folded along a portion shown by adashed line, as illustrated in FIG. 15C. Then, the outer portions of theexterior body 509 are bonded. For the bonding, thermocompression can beused, for example. At this time, an unbonded region (hereinafterreferred to as an inlet) is provided for part (or one side) of theexterior body 509 so that the electrolyte solution 508 can be introducedlater.

Next, the electrolyte solution 508 (not illustrated) is introduced intothe exterior body 509 through the inlet provided for the exterior body509. The electrolyte solution 508 is preferably introduced in a reducedpressure atmosphere or in an inert gas atmosphere. Lastly, the inlet isbonded. In the above manner, the laminated secondary battery 500 can bemanufactured.

When the positive electrode active material described in the aboveembodiment is used in the positive electrode 503, the secondary battery500 with high capacity and excellent cycle performance can be obtained.

[Bendable Secondary Battery]

Next, an example of a bendable secondary battery is described withreference to FIG. 16 and FIG. 17.

FIG. 16A is a schematic top view of a bendable secondary battery 250.FIG. 16B1, FIG. 16B2, and FIG. 16C are schematic cross-sectional viewsrespectively taken along the cutting line C1-C2, the cutting line C3-C4,and the cutting line A1-A2 in FIG. 16A. The secondary battery 250includes an exterior body 251 and an electrode stack 210 held in theexterior body 251. The electrode stack 210 has a structure in which atleast a positive electrode 211 a and a negative electrode 211 b arestacked. A lead 212 a electrically connected to the positive electrode211 a and a lead 212 b electrically connected to the negative electrode211 b are extended to the outside of the exterior body 251. In additionto the positive electrode 211 a and the negative electrode 211 b, anelectrolyte solution (not illustrated) is enclosed in a regionsurrounded by the exterior body 251.

The positive electrode 211 a and the negative electrode 211 b that areincluded in the secondary battery 250 are described using FIG. 17. FIG.17A is a perspective view illustrating the stacking order of thepositive electrode 211 a, the negative electrode 211 b, and a separator214. FIG. 17B is a perspective view illustrating the lead 212 a and thelead 212 b in addition to the positive electrode 211 a and the negativeelectrode 211 b.

As illustrated in FIG. 17A, the secondary battery 250 includes aplurality of strip-shaped positive electrodes 211 a, a plurality ofstrip-shaped negative electrodes 211 b, and a plurality of separators214. The positive electrode 211 a and the negative electrode 211 b eachinclude a projected tab portion and a portion other than the tabportion. A positive electrode active material layer is formed on onesurface of the positive electrode 211 a other than the tab portion, anda negative electrode active material layer is formed on one surface ofthe negative electrode 211 b other than the tab portion.

The positive electrodes 211 a and the negative electrodes 211 b arestacked so that surfaces of the positive electrodes 211 a on each ofwhich the positive electrode active material layer is not formed are incontact with each other and that surfaces of the negative electrodes 211b on each of which the negative electrode active material is not formedare in contact with each other.

Furthermore, the separator 214 is provided between the surface of thepositive electrode 211 a on which the positive electrode active materialis formed and the surface of the negative electrode 211 b on which thenegative electrode active material is formed. In FIG. 17, the separator214 is shown by a dotted line for clarity.

Furthermore, as illustrated in FIG. 17B, the plurality of positiveelectrodes 211 a are electrically connected to the lead 212 a in abonding portion 215 a. In addition, the plurality of negative electrodes211 b are electrically connected to the lead 212 b in a bonding portion215 b.

Next, the exterior body 251 is described using FIG. 16B1, FIG. 16B2,FIG. 16C, and FIG. 16D.

The exterior body 251 has a film-like shape and is folded in half so asto sandwich the positive electrodes 211 a and the negative electrodes211 b. The exterior body 251 includes a folded portion 261, a pair ofseal portions 262, and a seal portion 263. The pair of seal portions 262is provided with the positive electrodes 211 a and the negativeelectrodes 211 b sandwiched therebetween and thus can also be referredto as side seals. In addition, the seal portion 263 includes portionsoverlapping with the lead 212 a and the lead 212 b and can also bereferred to as a top seal.

Portions of the exterior body 251 that overlap with the positiveelectrodes 211 a and the negative electrodes 211 b preferably have awave shape in which crest lines 271 and trough lines 272 are alternatelyarranged. In addition, the seal portions 262 and the seal portion 263 ofthe exterior body 251 are preferably flat.

FIG. 16B1 is a cross section cut along a portion overlapping with thecrest line 271, and FIG. 16B2 is a cross section cut along a portionoverlapping with the trough line 272. FIG. 16B1 and FIG. 16B2 correspondto cross sections of the secondary battery 250, the positive electrodes211 a, and the negative electrodes 211 b in the width direction.

Here, the distance between the seal portion 262 and end portions of thepositive electrode 211 a and the negative electrode 211 b in the widthdirection, that is, the end portions of the positive electrode 211 a andthe negative electrode 211 b, is referred to as a distance La. When thesecondary battery 250 changes in shape, for example, is bent, thepositive electrode 211 a and the negative electrode 211 b change inshape such that the positions thereof are shifted from each other in thelength direction as described later. At the time, if the distance La istoo short, the exterior body 251 and the positive electrode 211 a andthe negative electrode 211 b are rubbed hard, so that the exterior body251 is damaged in some cases. In particular, when a metal film of theexterior body 251 is exposed, the metal film might be corroded by theelectrolyte solution. Therefore, the distance La is preferably set aslong as possible. However, if the distance La is too long, the volume ofthe secondary battery 250 is increased.

Furthermore, the distance La between the seal portion 262 and thepositive electrodes 211 a and the negative electrodes 211 b ispreferably increased as the total thickness of the stacked positiveelectrodes 211 a and negative electrodes 211 b is increased.

More specifically, when the total thickness of the stacked positiveelectrodes 211 a, negative electrodes 211 b, and separators 214 (notillustrated) is referred to as a thickness t, the distance La ispreferably 0.8 times or more and 3.0 times or less, further preferably0.9 times or more and 2.5 times or less, and still further preferably1.0 times or more and 2.0 times or less as large as the thickness t.When the distance La is in the above range, a compact battery that ishighly reliable for bending can be obtained.

Furthermore, when the distance between the pair of seal portions 262 isreferred to as a distance Lb, it is preferable that the distance Lb besufficiently longer than the widths of the positive electrode 211 a andthe negative electrode 211 b (here, a width Wb of the negative electrode211 b). Thus, even if the positive electrode 211 a and the negativeelectrode 211 b come into contact with the exterior body 251 due to achange in shape such as repeated bending of the secondary battery 250,parts of the positive electrode 211 a and the negative electrode 211 bcan be shifted in the width direction; thus, the positive electrode 211a and the negative electrode 211 b can be effectively prevented frombeing rubbed against the exterior body 251.

For example, the difference between the distance Lb, which is thedistance between the pair of seal portions 262, and the width Wb of thenegative electrode 211 b is preferably 1.6 times or more and 6.0 timesor less, further preferably 1.8 times or more and 5.0 times or less, andstill further preferably 2.0 times or more and 4.0 times or less aslarge as the thickness t of the positive electrode 211 a and thenegative electrode 211 b.

Furthermore, FIG. 16C illustrates a cross section including the lead 212a and corresponds to a cross section of the secondary battery 250, thepositive electrode 211 a, and the negative electrode 211 b in the lengthdirection. As illustrated in FIG. 16C, in the folded portion 261, aspace 273 is preferably included between the end portions of thepositive electrode 211 a and the negative electrode 211 b in the lengthdirection and the exterior body 251.

FIG. 16D is a schematic cross-sectional view of the secondary battery250 in a bent state. FIG. 16D corresponds to a cross section along thecutting line B1-B2 in FIG. 16A.

When the secondary battery 250 is bent, the exterior body 251 changes inshape such that part of the exterior body 251 positioned on the outerside in bending is stretched and the other part positioned on the innerside shrinks. More specifically, the part of the exterior body 251positioned on the outer side in bending changes in shape such that thewave amplitude becomes smaller and the wave period becomes longer. Incontrast, the part of the exterior body 251 positioned on the inner sidechanges in shape such that the wave amplitude becomes larger and thewave period becomes shorter. When the exterior body 251 changes in shapein this manner, stress applied to the exterior body 251 due to bendingis relieved, so that a material itself of the exterior body 251 does notneed to expand and contract. As a result, the secondary battery 250 canbe bent with weak force without damage to the exterior body 251.

Furthermore, as illustrated in FIG. 16D, when the secondary battery 250is bent, the positive electrode 211 a and the negative electrode 211 bare relatively shifted to each other. At this time, ends of the stackedpositive electrodes 211 a and negative electrodes 211 b on the sealportion 263 side are fixed by a fixing member 217; thus, the pluralityof positive electrodes 211 a and negative electrodes 211 b are shiftedso that the shift amount becomes larger at a position closer to thefolded portion 261. Therefore, stress applied to the positive electrode211 a and the negative electrode 211 b is relieved, and the positiveelectrode 211 a and the negative electrode 211 b themselves do not needto expand and contract. As a result, the secondary battery 250 can bebent without damage to the positive electrode 211 a and the negativeelectrode 211 b.

Furthermore, the space 273 is included between the exterior body 251 andthe positive electrode 211 a and the negative electrode 211 b, wherebythe positive electrode 211 a and the negative electrode 211 b can beshifted relatively while the positive electrode 211 a and the negativeelectrode 211 b positioned on an inner side in bending do not come incontact with the exterior body 251.

In the secondary battery 250 illustrated in FIG. 16 and FIG. 17, damageto the exterior body, damage to the positive electrode 211 a and thenegative electrode 211 b, and the like are less likely to occur andbattery characteristics are less likely to deteriorate even when thesecondary battery 250 is repeatedly bent and stretched. When thepositive electrode active material described in the above embodiment isused in the positive electrode 211 a included in the secondary battery250, a battery with better cycle performance can be obtained.

Embodiment 5

In this embodiment, examples of electronic devices each including thesecondary battery of one embodiment of the present invention aredescribed.

First, FIG. 18A to FIG. 18G illustrates examples of electronic devicesincluding the bendable secondary battery described in part of Embodiment3. Examples of electronic devices each including a bendable secondarybattery include television sets (also referred to as televisions ortelevision receivers), monitors of computers or the like, digitalcameras, digital video cameras, digital photo frames, mobile phones(also referred to as cellular phones or mobile phone devices), portablegame machines, portable information terminals, audio reproducingdevices, and large game machines such as pachinko machines.

Furthermore, a secondary battery with a flexible shape can also beincorporated along a curved surface of an inside wall or an outside wallof a house or a building or an interior or an exterior of an automobile.

FIG. 18A illustrates an example of a mobile phone. A mobile phone 7400includes operation buttons 7403, an external connection port 7404, aspeaker 7405, a microphone 7406, and the like in addition to a displayportion 7402 incorporated in a housing 7401. Note that the mobile phone7400 includes a secondary battery 7407. When the secondary battery ofone embodiment of the present invention is used as the secondary battery7407, a lightweight mobile phone with a long lifetime can be provided.

FIG. 18B illustrates the mobile phone 7400 in a bent state. When thewhole mobile phone 7400 is bent through deformation by the externalforce, the secondary battery 7407 provided therein is also bent. FIG.18C illustrates the bent secondary battery 7407 in a bent state. Thesecondary battery 7407 is a thin storage battery. The secondary battery7407 is fixed in a bent state. Note that the secondary battery 7407includes a lead electrode electrically connected to a current collector.The current collector is, for example, copper foil and partly alloyedwith gallium; thus, adhesion between the current collector and an activematerial layer in contact with the current collector is improved and thesecondary battery 7407 can have high reliability even in a bent state.

FIG. 18D illustrates an example of a bangle-type display device. Aportable display device 7100 includes a housing 7101, a display portion7102, operation buttons 7103, and a secondary battery 7104. FIG. 18Eillustrates the secondary battery 7104 in a bent state. When the displaydevice is worn on a user's arm while the secondary battery 7104 is bent,the housing changes in shape and the curvature of part or the whole ofthe secondary battery 7104 is changed. Note that a value represented bythe radius of a circle that corresponds to the bending degree of a curveat a given point is referred to as the radius of curvature, and thereciprocal of the radius of curvature is referred to as curvature.Specifically, part or the whole of the housing or the main surface ofthe secondary battery 7104 is changed in the range of radius ofcurvature from 40 mm or more to 150 mm or less. When the radius ofcurvature at the main surface of the secondary battery 7104 is in therange from 40 mm or more to 150 mm or less, the reliability can be kepthigh. When the secondary battery of one embodiment of the presentinvention is used as the secondary battery 7104, a lightweight portabledisplay device with a long lifetime can be provided.

FIG. 18F illustrates an example of a watch-type portable informationterminal. A portable information terminal 7200 includes a housing 7201,a display portion 7202, a band 7203, a buckle 7204, an operation button7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music replay, Internet communication, and a computergame.

The display portion 7202 with a curved display surface is provided, andimages can be displayed on the curved display surface. In addition, thedisplay portion 7202 includes a touch sensor, and operation can beperformed by touching the screen with a finger, a stylus, or the like.For example, by touching an icon 7207 displayed on the display portion7202, application can be started.

With the operation button 7205, a variety of functions such as timesetting, power on/off, on/off of wireless communication, setting andcancellation of a silent mode, and setting and cancellation of a powersaving mode can be performed. For example, the functions of theoperation button 7205 can be set freely with the operating systemincorporated in the portable information terminal 7200.

Furthermore, the portable information terminal 7200 can execute nearfield communication that is standardized communication. For example,mutual communication with a headset capable of wireless communicationenables hands-free calling.

Furthermore, the portable information terminal 7200 includes the inputoutput terminal 7206, and can perform direct data communication withanother information terminal via a connector. In addition, charging viathe input output terminal 7206 is possible. Note that the chargingoperation may be performed by wireless power feeding without using theinput output terminal 7206.

The display portion 7202 of the portable information terminal 7200includes the secondary battery of one embodiment of the presentinvention. When the secondary battery of one embodiment of the presentinvention is used, a lightweight portable information terminal with along lifetime can be provided. For example, the secondary battery 7104illustrated in FIG. 18E can be provided in the housing 7201 while beingcurved, or can be provided in the band 7203 such that it can be curved.

The portable information terminal 7200 preferably includes a sensor. Asthe sensor, for example, a human body sensor such as a fingerprintsensor, a pulse sensor, or a temperature sensor; a touch sensor; apressure sensitive sensor; an acceleration sensor; or the like ispreferably mounted.

FIG. 18G illustrates an example of an armband display device. A displaydevice 7300 includes a display portion 7304 and the secondary battery ofone embodiment of the present invention. In addition, the display device7300 can include a touch sensor in the display portion 7304 and canserve as a portable information terminal.

The display surface of the display portion 7304 is curved, and imagescan be displayed on the curved display surface. A display state of thedisplay device 7300 can be changed by, for example, near fieldcommunication that is standardized communication.

Furthermore, the display device 7300 includes an input/output terminal,and can perform direct data communication with another informationterminal via a connector. In addition, charging via the input/outputterminal is possible. Note that the charging operation may be performedby wireless power feeding without using the input/output terminal.

When the secondary battery of one embodiment of the present invention isused as the secondary battery included in the display device 7300, alightweight display device with a long lifetime can be provided.

Examples of electronic devices each including the secondary battery withexcellent cycle performance described in the above embodiment aredescribed using FIG. 18H, FIG. 19, and FIG. 20.

When the secondary battery of one embodiment of the present invention isused as a secondary battery of a daily electronic device, a lightweightproduct with a long lifetime can be provided. Examples of the dailyelectronic device include an electric toothbrush, an electric shaver,and electric beauty equipment. As secondary batteries of these products,small and lightweight secondary batteries with stick-like shapes andhigh capacity are desired in consideration of handling ease for users.

FIG. 18H is a perspective view of a device called a cigarette smokingdevice (electronic cigarette). In FIG. 18H, an electronic cigarette 7500includes an atomizer 7501 including a heating element, a secondarybattery 7504 that supplies power to the atomizer, and a cartridge 7502including a liquid supply bottle, a sensor, and the like. To improvesafety, a protection circuit that prevents overcharging andoverdischarging of the secondary battery 7504 may be electricallyconnected to the secondary battery 7504. The secondary battery 7504 inFIG. 18H includes an external terminal for connection to a charger. Whenthe electronic cigarette 7500 is held by a user, the secondary battery7504 becomes a tip portion; thus, it is preferable that the secondarybattery 7504 have a short total length and be lightweight. With thesecondary battery of one embodiment of the present invention, which hashigh capacity and excellent cycle performance, the small and lightweightelectronic cigarette 7500 that can be used for a long time over a longperiod can be provided.

Next, FIG. 19A and FIG. 19B illustrate an example of a tablet terminalthat can be folded in half. A tablet terminal 9600 illustrated in FIG.19A and FIG. 19B includes a housing 9630 a, a housing 9630 b, a movableportion 9640 connecting the housing 9630 a and the housing 9630 b toeach other, a display portion 9631 including a display portion 9631 aand a display portion 9631 b, a switch 9625, a switch 9626, a switch9627, a fastener 9629, and an operation switch 9628. By using a flexiblepanel for the display portion 9631, the tablet terminal can have alarger display portion. FIG. 19A illustrates the tablet terminal 9600that is opened, and FIG. 19B illustrates the tablet terminal 9600 thatis closed.

Furthermore, the tablet terminal 9600 includes a power storage unit 9635inside the housing 9630 a and the housing 9630 b. The power storage unit9635 is provided across the housing 9630 a and the housing 9630 b,passing through the movable portion 9640.

Part of or the entire display portion 9631 can be a touch panel region,and data can be input by touching text, an input form, an imageincluding an icon, and the like displayed on the region. For example, itis possible that keyboard buttons are displayed on the entire displayportion 9631 a on the housing 9630 a side, and data such as text or animage is displayed on the display portion 9631 b on the housing 9630 bside.

Furthermore, it is possible that a keyboard is displayed on the displayportion 9631 b on the housing 9630 b side, and data such as text or animage is displayed on the display portion 9631 a on the housing 9630 aside. Furthermore, it is possible that a button for switching keyboarddisplay on a touch panel is displayed on the display portion 9631 andthe button is touched with a finger, a stylus, or the like to display akeyboard on the display portion 9631.

Furthermore, touch input can be performed concurrently in a touch panelregion in the display portion 9631 a on the housing 9630 a side and atouch panel region in the display portion 9631 b on the housing 9630 bside.

Furthermore, the switch 9625 to the switch 9627 may function not only asan interface for operating the tablet terminal 9600 but also as aninterface that can switch various functions. For example, at least oneof the switch 9625 to the switch 9627 may have a function of switchingpower on/off of the tablet terminal 9600. For another example, at leastone of the switch 9625 to the switch 9627 may have a function ofswitching display between a portrait mode and a landscape mode and afunction of switching display between monochrome display and colordisplay. For another example, at least one of the switch 9625 to theswitch 9627 may have a function of adjusting the luminance of thedisplay portion 9631. The luminance of the display portion 9631 can beoptimized in accordance with the amount of external light in use of thetablet terminal 9600, which is detected by an optical sensorincorporated in the tablet terminal 9600. Note that another sensingdevice including a sensor for measuring inclination, such as a gyroscopesensor or an acceleration sensor, may be incorporated in the tabletterminal, in addition to the optical sensor.

Furthermore, although FIG. 19A illustrates the example where the displayportion 9631 a on the housing 9630 a side and the display portion 9631 bon the housing 9630 b side have substantially the same display area,there is no particular limitation on the display area of each of thedisplay portion 9631 a and the display portion 9631 b; the size may bedifferent between one and the other, and the display quality may also bedifferent. For example, one may be a display panel that can displayhigher-definition images than the other.

The tablet terminal 9600 is folded in half in FIG. 19B, and the tabletterminal 9600 includes a housing 9630, a solar cell 9633, and a chargeand discharge control circuit 9634 including a DCDC converter 9636. Thepower storage unit of one embodiment of the present invention is used asthe power storage unit 9635.

Note that as described above, the tablet terminal 9600 can be folded inhalf, and thus can be folded when not in use such that the housing 9630a and the housing 9630 b overlap with each other. The display portion9631 can be protected owing to the folding, which can increase thedurability of the tablet terminal 9600. Since the power storage unit9635 including the secondary battery of one embodiment of the presentinvention has high capacity and excellent cycle performance, the tabletterminal 9600 that can be used for a long time over a long period can beprovided.

Furthermore, the tablet terminal 9600 illustrated in FIG. 19A and FIG.19B can also have a function of displaying various kinds of information(a still image, a moving image, a text image, and the like), a functionof displaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing data displayedon the display portion by touch input, a function of controllingprocessing by various kinds of software (programs), and the like.

The solar cell 9633 attached on the surface of the tablet terminal 9600supplies electric power to a touch panel, a display portion, a videosignal processing portion, and the like. Note that the solar cell 9633can be provided on one surface or both surfaces of the housing 9630 andthe power storage unit 9635 can be charged efficiently. Note that theuse of a lithium-ion battery as the power storage unit 9635 brings anadvantage such as a reduction in size.

Furthermore, the structure and operation of the charge and dischargecontrol circuit 9634 illustrated in FIG. 19B are described withreference to a block diagram in FIG. 19C. The solar cell 9633, the powerstorage unit 9635, the DCDC converter 9636, a converter 9637, switchesSW1, SW2, and SW3, and the display portion 9631 are illustrated in FIG.19C, and the power storage unit 9635, the DCDC converter 9636, theconverter 9637, and the switches SW1 to SW3 correspond to the charge anddischarge control circuit 9634 illustrated in FIG. 19B.

First, an operation example in the case where electric power isgenerated by the solar cell 9633 using external light is described. Thevoltage of electric power generated by the solar cell is raised orlowered by the DCDC converter 9636 to a voltage for charging the powerstorage unit 9635. Then, when the display portion 9631 is operated withthe electric power from the solar cell 9633, the switch SW1 is turned onand the voltage is raised or lowered by the converter 9637 so as to be avoltage needed for the display portion 9631. When display on the displayportion 9631 is not performed, the switch SW1 is turned off and theswitch SW2 is turned on so that the power storage unit 9635 is charged.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the power storage unit 9635 may be charged with anotherpower generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thepower storage unit 9635 may be charged with a non-contact powertransmission module that transmits and receives power wirelessly(without contact) for charging, or with a combination of other chargingmeans.

FIG. 20 illustrates other examples of electronic devices. In FIG. 20, adisplay device 8000 is an example of an electronic device using asecondary battery 8004 of one embodiment of the present invention.Specifically, the display device 8000 corresponds to a display devicefor TV broadcast reception and includes a housing 8001, a displayportion 8002, speaker portions 8003, the secondary battery 8004, and thelike. The secondary battery 8004 of one embodiment of the presentinvention is provided in the housing 8001. The display device 8000 canreceive electric power supply from a commercial power source or can useelectric power stored in the secondary battery 8004. Thus, the displaydevice 8000 can be used with the use of the secondary battery 8004 ofone embodiment of the present invention as an uninterruptible powersupply even when electric power supply cannot be received from acommercial power source due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a DMD (Digital Micromirror Device), a PDP (Plasma DisplayPanel), or an FED (Field Emission Display) can be used for the displayportion 8002.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like other than TV broadcast reception.

In FIG. 20, an installation lighting device 8100 is an example of anelectronic device using a secondary battery 8103 of one embodiment ofthe present invention. Specifically, the lighting device 8100 includes ahousing 8101, a light source 8102, the secondary battery 8103, and thelike. Although FIG. 20 illustrates the case where the secondary battery8103 is provided in a ceiling 8104 on which the housing 8101 and thelight source 8102 are installed, the secondary battery 8103 may beprovided in the housing 8101. The lighting device 8100 can receiveelectric power supply from a commercial power source, or can useelectric power stored in the secondary battery 8103. Thus, the lightingdevice 8100 can be used with the use of the secondary battery 8103 ofone embodiment of the present invention as an uninterruptible powersupply even when electric power supply cannot be received from acommercial power source due to power failure or the like.

Note that although the installation lighting device 8100 provided in theceiling 8104 is illustrated in FIG. 20, the secondary battery of oneembodiment of the present invention can be used for an installationlighting device provided in, for example, a wall 8105, a floor 8106, awindow 8107, or the like other than the ceiling 8104, or can be used ina tabletop lighting device or the like.

Furthermore, as the light source 8102, an artificial light source thatobtains light artificially by using power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, and alight-emitting element such as an LED or an organic EL element are givenas examples of the artificial light source.

In FIG. 20, an air conditioner including an indoor unit 8200 and anoutdoor unit 8204 is an example of an electronic device using asecondary battery 8203 of one embodiment of the present invention.Specifically, the indoor unit 8200 includes a housing 8201, an airoutlet 8202, the secondary battery 8203, and the like. Although FIG. 20illustrates the case where the secondary battery 8203 is provided in theindoor unit 8200, the secondary battery 8203 may be provided in theoutdoor unit 8204. Alternatively, the secondary battery 8203 may beprovided in both the indoor unit 8200 and the outdoor unit 8204. The airconditioner can receive electric power supply from a commercial powersource, or can use electric power stored in the secondary battery 8203.Particularly in the case where the secondary battery 8203 is provided inboth the indoor unit 8200 and the outdoor unit 8204, the air conditionercan be operated with the use of the secondary battery 8203 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power supply cannot be received from a commercialpower source due to power failure or the like.

Note that although FIG. 20 illustrates the split-type air conditionercomposed of the indoor unit and the outdoor unit, the secondary batteryof one embodiment of the present invention can also be used in anintegrated air conditioner in which one housing has the function of anindoor unit and the function of an outdoor unit.

In FIG. 20, an electric refrigerator-freezer 8300 is an example of anelectronic device using a secondary battery 8304 of one embodiment ofthe present invention. Specifically, the electric refrigerator-freezer8300 includes a housing 8301, a door for refrigerator compartment 8302,a door for freezer compartment 8303, the secondary battery 8304, and thelike. The secondary battery 8304 is provided in the housing 8301 in FIG.20. The electric refrigerator-freezer 8300 can receive electric powersupply from a commercial power source, or can use electric power storedin the secondary battery 8304. Thus, the electric refrigerator-freezer8300 can be used with the use of the secondary battery 8304 of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power supply cannot be received from a commercialpower source due to power failure or the like.

Note that among the electronic devices described above, a high-frequencyheating apparatus such as a microwave oven and an electronic device suchas an electric rice cooker require high power in a short time.Therefore, the tripping of a breaker of a commercial power source in useof an electronic device can be prevented by using the secondary batteryof one embodiment of the present invention as an auxiliary power supplyfor supplying electric power which cannot be supplied enough by acommercial power source.

In addition, in a time period when electronic devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power source (such a proportion referred toas a usage rate of electric power) is low, electric power can be storedin the secondary battery, whereby an increase in the usage rate ofelectric power can be reduced in a time period when the electronicdevices are used. For example, in the case of the electricrefrigerator-freezer 8300, electric power can be stored in the secondarybattery 8304 in night time when the temperature is low and the door forrefrigerator compartment 8302 and the door for freezer compartment 8303are not opened and closed. Moreover, in daytime when the temperature ishigh and the door for refrigerator compartment 8302 and the door forfreezer compartment 8303 are opened and closed, the usage rate of powerin daytime can be kept low by using the secondary battery 8304 as anauxiliary power supply.

According to one embodiment of the present invention, the secondarybattery can have excellent cycle performance and improved reliability.Furthermore, according to one embodiment of the present invention, asecondary battery with high capacity can be obtained; thus, thesecondary battery itself can be made more compact and lightweight owingto the improvement in the characteristics of the secondary battery.Thus, the secondary battery of one embodiment of the present inventionis incorporated in the electronic device described in this embodiment,whereby a more lightweight electronic device with a longer lifetime canbe obtained. This embodiment can be implemented in combination with anyof the other embodiments as appropriate.

Embodiment 6

In this embodiment, examples of vehicles each including the secondarybattery of one embodiment of the present invention are described.

The use of secondary batteries in vehicles enables production ofnext-generation clean energy vehicles such as hybrid electric vehicles(HVs), electric vehicles (EVs), and plug-in hybrid electric vehicles(PHVs).

FIG. 21 illustrates examples of vehicles each including the secondarybattery of one embodiment of the present invention. An automobile 8400illustrated in FIG. 21A is an electric vehicle that runs on an electricmotor as a power source. Alternatively, the automobile 8400 is a hybridelectric vehicle capable of driving using either an electric motor or anengine with an appropriate selection. The use of one embodiment of thepresent invention can achieve a high-mileage vehicle. The automobile8400 includes the secondary battery. As the secondary battery, themodules of the secondary batteries illustrated in FIG. 6C and FIG. 6Dmay be arranged to be used in a floor portion in the automobile.Alternatively, a battery pack in which a plurality of secondarybatteries illustrated in FIG. 9 are combined may be placed in the floorportion in the automobile. The secondary battery is used not only fordriving an electric motor 8406, but also for supplying electric power toa light-emitting device such as a headlight 8401 or a room light (notillustrated).

Furthermore, the secondary battery can also supply electric power to adisplay device included in the automobile 8400, such as a speedometer ora tachometer. Furthermore, the secondary battery can supply electricpower to a semiconductor device included in the automobile 8400, such asa navigation system.

An automobile 8500 illustrated in FIG. 21B can be charged when asecondary battery included in the automobile 8500 is supplied with powerthrough external charging equipment by a plug-in system, a contactlesspower feeding system, or the like. FIG. 21B illustrates a state where asecondary battery 8024 incorporated in the automobile 8500 is chargedfrom a ground installation type charging apparatus 8021 through a cable8022. In charging, a given method such as CHAdeMO (registered trademark)or Combined Charging System may be employed as a charging method, thestandard of a connector, or the like as appropriate. The chargingapparatus 8021 may be a charging station provided in a commerce facilityor a power source in a house. For example, with a plug-in technique, thesecondary battery 8024 incorporated in the automobile 8500 can becharged by power supply from the outside. The charging can be performedby converting AC electric power into DC electric power through aconverter, such as an AC-DC converter.

Although not illustrated, the vehicle may include a power receivingdevice so that it can be charged by being supplied with electric powerfrom an above-ground power transmitting device in a contactless manner.In the case of the contactless power feeding system, by incorporating apower transmitting device in a road or an exterior wall, charging can beperformed not only when the electric vehicle is stopped but also whendriven. In addition, the contactless power feeding system may beutilized to perform transmission and reception of electric power betweenvehicles. Furthermore, a solar cell may be provided in the exterior ofthe vehicle to charge the secondary battery while the vehicle is stoppedor driven. To supply electric power in such a contactless manner, anelectromagnetic induction method or a magnetic resonance method can beused.

Furthermore, FIG. 21C is an example of a motorcycle including thesecondary battery of one embodiment of the present invention. A scooter8600 illustrated in FIG. 21C includes a secondary battery 8602, sidemirrors 8601, and direction indicators 8603. The secondary battery 8602can supply electricity to the direction indicators 8603.

Furthermore, in the scooter 8600 illustrated in FIG. 21C, the secondarybattery 8602 can be stored in an under-seat storage 8604. The secondarybattery 8602 can be stored in the under-seat storage 8604 even when theunder-seat storage 8604 is small. The secondary battery 8602 isdetachable; thus, the secondary battery 8602 is carried indoors whencharged, and is stored before the motor scooter is driven.

According to one embodiment of the present invention, the secondarybattery can have improved cycle performance and the secondary batterycan have higher capacity. Thus, the secondary battery itself can be mademore compact and lightweight. The compact and lightweight secondarybattery contributes to a reduction in the weight of a vehicle, and thusincreases the mileage. Furthermore, the secondary battery included inthe vehicle can be used as a power supply source for products other thanthe vehicle. In such a case, the use of a commercial power source can beavoided at peak time of electric power demand, for example. Avoiding theuse of a commercial power source at peak time of electric power demandcan contribute to energy saving and a reduction in carbon dioxideemissions. Moreover, the secondary battery with excellent cycleperformance can be used over a long period; thus, the use amount of raremetals such as cobalt can be reduced.

This embodiment can be implemented in combination with the otherembodiments as appropriate.

Embodiment 7

In this embodiment, examples of wearable devices in which a secondarybattery including the positive electrode active material of oneembodiment of the present invention can be provided are described.

FIG. 22A illustrates examples of wearable devices. A secondary batteryis used as a power source of a wearable device. To have improved waterresistance in daily use or outdoor use by a user, a wearable device isdesirably capable of being charged wirelessly as well as being chargedwith a wire whose connector portion for connection is exposed.

For example, a secondary battery can be incorporated in a glasses-typedevice 400 as illustrated in FIG. 22A. The glasses-type device 400includes a frame 400 a and a display portion 400 b. A secondary batteryis incorporated in a temple of the frame 400 a having a curved shape,whereby the glasses-type device 400 can be lightweight, have awell-balanced weight, and be used continuously for a long time.

Furthermore, the secondary battery can be incorporated in a headset-typedevice 401.

The headset-type device 401 includes at least a microphone portion 401a, a flexible pipe 401 b, and an earphone portion 401 c. The secondarybattery can be provided in the flexible pipe 401 b or the earphoneportion 401 c.

The secondary battery can also be incorporated in a device 402 that canbe directly attached to a human body. A secondary battery 402 b can beprovided in a thin housing 402 a of the device 402.

The secondary battery can be incorporated in a device 403 that can beattached to clothing. A secondary battery 403 b can be provided in athin housing 403 a of the device 403.

Furthermore, the secondary battery can be incorporated in a belt-typedevice 406. The belt-type device 406 includes a belt portion 406 a and awireless power feeding and receiving portion 406 b, and the secondarybattery can be incorporated in the belt portion 406 a.

The secondary battery can also be incorporated in a watch-type device405. The watch-type device 405 includes a display portion 405 a and abelt portion 405 b, and the secondary battery can be provided in thedisplay portion 405 a or the belt portion 405 b.

The display portion 405 a can display various kinds of information suchas reception information of an e-mail or an incoming call in addition totime.

Since the watch-type device 405 is a type of wearable device that isdirectly wrapped around an arm, a sensor that measures pulse, bloodpressure, or the like of a user can be incorporated therein. Data on theexercise quantity and health of the user can be stored to be used forhealth maintenance.

A watch-type device 405 illustrated in FIG. 22A is described in detailbelow.

FIG. 22B illustrates a perspective view of the watch-type device 405that is detached from an arm.

FIG. 22C illustrates a side view. FIG. 22C illustrates a state where thesecondary battery 913 is incorporated in the watch-type device 405. Thesecondary battery 913, which is small and lightweight, is provided at aposition overlapping with the display portion 405 a.

This embodiment can be implemented in appropriate combination with theother embodiments.

Example 1

In this example, materials used in forming positive electrode activematerials were inspected using the inspection by DSC described in theabove embodiment.

Using the method illustrated in FIG. 1A, Sample 1 which is a mixture ofthe material 91 and the material 92 was formed and inspected. As thematerial 91 and the material 92, lithium fluoride and magnesium fluoridewere used, respectively. The molar ratio of lithium included in lithiumfluoride was set to 0.33 times that of magnesium included in magnesiumfluoride. DSC was used as the inspection method.

Next, using the method illustrated in FIG. 1C, the material 91 to thematerial 94 were inspected. As the material 91, the material 92, thematerial 93, and the material 94, lithium fluoride, magnesium fluoride,nickel hydroxide, and aluminum hydroxide were used, respectively. Amixture of the material 91, the material 92, the material 93, and thematerial 94 is formed as Sample 2. In Sample 2, the molar ratio oflithium included in lithium fluoride, the molar ratio of nickel includedin nickel hydroxide, and the molar ratio of aluminum included inaluminum hydroxide were set to 0.33 times, 0.5 times, and 0.5 times,respectively, that of magnesium included in magnesium fluoride. DSC wasused as the inspection method.

Temperature-heat flow curves of Sample 1 and Sample 2 by DSC are shownin FIG. 23A. Sample 1 had a peak suggesting an endothermic reaction inthe vicinity of 730° C. in the observed result, from which it isconceivable that a eutectic reaction occurs between lithium fluoride andmagnesium fluoride. In contrast, Sample 2 does not show a significantpeak suggesting an endothermic reaction in the vicinity of 730° C.Therefore, either the material 93 or the material 94 is probably likelyto inhibit the endothermic reaction.

FIG. 23B and FIG. 23C show a differential waveform of thetemperature-heat flow curve of Sample 1 by DSC and a differentialwaveform of the temperature-heat flow curve of Sample 2 by DSC,respectively. Sample 1 had a local maximum point and a local minimumpoint in the vicinity of 730° C. where the peak was observed in thetemperature-heat flow curve. In contrast, Sample 2 showed no significantpeak or an extremely weak peak.

Next, a mixture of Sample 1 and nickel hydroxide was formed as Sample 3,and a mixture of Sample 1 and aluminum hydroxide was formed as Sample 4.Each of the samples was subjected to DSC. In Sample 3, the molar ratioof nickel included in nickel hydroxide was set to 0.5 times that ofmagnesium included in magnesium fluoride. In Sample 4, the molar ratioof aluminum included in aluminum hydroxide was set to 0.5 times that ofmagnesium included in magnesium fluoride. A differential waveform ofSample 3 by DSC and a differential waveform of Sample 4 by DSC are shownin FIG. 24A and FIG. 24B, respectively. A peak suggesting an endothermicreaction in the vicinity of 730° C. is smaller in Sample 4 than inSample 3. In addition, in Sample 4, a peak suggesting an exothermicreaction is observed in the vicinity of 500° C., which shows apossibility of a reaction such as crystallization or generation of acompound by a reaction with magnesium or the like. An inhibitory factorof the endothermic reaction between lithium fluoride and magnesiumfluoride is conceivably aluminum hydroxide.

It was suggested that a positive electrode active material with higherquality can be obtained in the case where the method of FIG. 2B or FIG.2C is used as the method for forming a positive electrode activematerial, compared with the case where the method of FIG. 2A is used.

Next, lithium cobalt oxide serving as the metal oxide 95 was furtheradded to and mixed with the mixture of the material 91 and the material92 (Sample 1) to form Sample 5. FIG. 25 shows a differential waveform ofSample 5 by DSC. It was found that the addition of lithium cobalt oxidepositively shifts the peak suggesting the endothermic reaction observedby DSC, by approximately 100° C. from the vicinity of 730° C.

Example 2

In this example, positive electrode active materials were formed usingthe formation methods described in the above embodiment, and secondarybatteries were fabricated in order to evaluate characteristics ofpositive electrodes using the formed positive electrode activematerials.

Three secondary batteries, which were Cell 1, Cell 2, and Cell 3, werefabricated.

As the positive electrode active material used in Cell 1, the positiveelectrode active material was formed using the method illustrated inFIG. 2B. More specifically, the method illustrated in FIG. 3 was used.As the material 91 and the material 92, lithium fluoride and magnesiumfluoride were used, respectively. As for the molar ratio between thematerial 91 and the material 92, Sample 1 was referred to. The weight ofthe powder of the mixture 904 was 30 g.

As the positive electrode active materials used in Cell 2 and Cell 3,the positive electrode active material was formed using the methodillustrated in FIG. 2A. As the material 91, the material 92, thematerial 93, and the material 94, lithium fluoride, magnesium fluoride,nickel hydroxide, and aluminum hydroxide were used, respectively. As forthe molar ratio between the material 91 and the material 92, Sample 1was referred to. The molar ratio of nickel included in nickel hydroxidewas set to 0.5 times that of magnesium included in magnesium fluoride,and the molar ratio of aluminum included in aluminum hydroxide was setto 0.5 times that of magnesium included in magnesium fluoride. The totalweight of the powders of the material 91 to the material 94 was 30 g inCell 2 and 2.4 g in Cell 3.

As Cell 1, Cell 2, and Cell 3, CR2032 coin-type secondary batteries(diameter: 20 mm, height: 3.2 mm) were fabricated.

A positive electrode formed by applying, to a current collector, slurryin which the positive electrode active material formed in the abovemanner, acetylene black (AB), and polyvinylidene fluoride (PVDF) weremixed at the positive electrode active material: AB:PVDF=95:3:2 (weightratio) was used.

A lithium metal was used as a counter electrode.

As an electrolyte included in an electrolyte solution, 1 mol/L lithiumhexafluorophosphate (LiPF₆) was used, and as the electrolyte solution,an electrolyte solution in which ethylene carbonate (EC) and diethylcarbonate (DEC) at EC:DEC=3:7 (volume ratio) and vinylene carbonate (VC)at 2 wt % were mixed was used.

As a separator, 25-μm-thick polypropylene was used.

A positive electrode can and a negative electrode can formed usingstainless steel (SUS) were used.

[Cycle Performance]

The CCCV charge (0.5 C, 4.6 V, termination current: 0.05 C) and the CCdischarge (0.5 C, 2.5 V) were repeatedly performed on the fabricatedsecondary batteries at 45° C., and the cycle performance was evaluated.

FIG. 26 shows the cycle performance result for each of the secondarybatteries. When focusing on the aluminum hydroxide whose tendency toinhibit the eutectic reaction between lithium fluoride and magnesiumfluoride was suggested, Cell 1, which includes the positive electrodeactive material formed by the formation method of FIG. 3, that is, theformation method where aluminum hydroxide is added at the time offorming the mixture to be subjected to the second annealing, showedcycle performance superior to the cell formed using the formation methodwhere aluminum hydroxide is added at the first annealing. Furthermore,even in the case where aluminum hydroxide was added at the firstannealing, when the weight of the powder at the time of annealing waslight, superior performance was obtained.

Example 3

In this example, materials used in forming positive electrode activematerials were inspected using the inspection by XRD described in theabove embodiment.

Sample 2, Sample 3, and Sample 4 fabricated in the above example wereeach subjected to heat treatment at 850° C. in an oxygen atmosphere, andthen evaluation was performed by XRD. The XRD spectra are shown in FIG.27, FIG. 28, FIG. 29, and FIG. 30. In FIG. 27 to FIG. 30, the verticalaxis represents the intensity of the spectra (INTENSITY), and thehorizontal axis represents 2θ. The graphs in FIG. 27 to FIG. 30 havediffering range of 2θ, which is indicated by the horizontal axis.

For the XRD measurement, D8 ADVANCE manufactured by Bruker Corporationwas used. XRD measurement was performed under the following measurementconditions where the X-ray output was 40 kV, 40 mA, the range of thescanning angle was 15° to 90°, the measurement interval was 0.01°, thescanning rate was 0.5 sec/step, and the sample was rotated at 15 rpm.

On the obtained XRD patterns, background removal and Kα2 removal wereperformed using DIFFRAC.EVA (XRD data analysis software manufactured byBruker Corporation).

The peak positions, half widths, and peak intensities of the main peaksbased on the obtained results of XRD on Sample 2 are shown in Table 1.Here, the peak position refers to the maximum value of the peak.Although not shown in Table 1, an intensity of approximately 37 wasobserved at 37.03° position.

TABLE 1 peak position half width peak intensity (°) (°) (counts persecond) 19.08 0.406 6.63 27.30 0.076 168 31.44 0.307 5.9 35.23 0.08726.9 37.25 0.149 94 38.53 0.261 5.76 40.43 0.088 138 43.28 0.122 15643.79 0.091 43 45.03 0.369 29.5 53.53 0.103 100 56.29 0.099 40 59.600.435 7.96 60.65 0.122 20.6 62.86 0.208 52.6 63.64 0.128 5.87 64.680.078 4.61 65.57 0.41 26.5 67.74 0.113 14.4 68.16 0.132 36.5 71.57 0.1495.75 75.38 0.217 17.5 77.77 0.175 8.27 79.36 0.243 12.6 83.11 0.358 3.3483.67 0.102 4.63 86.85 0.148 3.67 87.38 0.151 10.3

Peaks at 2θ of 19.08°, 31.44°, 59.60°, and the like that probablycorrespond to MgAl_((2-x))Ni_(x)O₄ (x is greater than or equal to 0 andless than or equal to 2) with a spinel structure were observed fromSample 2. These peaks had peak intensities that were 0.05 times, 0.04times, and 0.06 times that of the peak at 40.43°.

From the results in FIG. 27 to FIG. 30 and Table 1, it is probable thata product was generated by a reaction between magnesium in magnesiumfluoride and aluminum in aluminum hydroxide. In addition, thisgeneration of the product may have possibly weaken the eutectic reactionbetween lithium fluoride and magnesium fluoride.

Next, XRD measurement was performed on the positive electrode activematerial used in Cell 2 described in Example 2. The XRD spectra areshown in FIG. 43A, FIG. 43B, FIG. 44A, and FIG. 44B. In FIG. 43A, FIG.43B, FIG. 44A, and FIG. 44B, the vertical axis represents the intensityof the spectrum (INTENSITY), and the horizontal axis represents 2θ. Thegraphs in FIG. 43A, FIG. 43B, FIG. 44A, and FIG. 44B have differingrange of 2θ, which is indicated by the horizontal axis.

For the XRD measurement, D8 ADVANCE manufactured by Bruker Corporationwas used. XRD measurement was performed under the following measurementconditions where the X-ray output was 40 kV, 40 mA, the range of thescanning angle was 15° to 90°, the measurement interval was 0.01°, thescanning rate was 5 sec/step, and the sample was rotated at 15 rpm.

On the obtained XRD patterns, background removal and Kα2 removal wereperformed using DIFFRAC.EVA (XRD data analysis software manufactured byBruker Corporation).

The peak positions, half widths, and peak intensities of the main peaksbased on the obtained results of XRD are shown in Table 2. Here, thepeak position refers to the maximum value of the peak.

TABLE 2 peak position half width peak intensity (°) (°) (count persecond) 18.966 0.046 4105 37.43 0.05 173 38.434 0.044 273 39.099 0.0568.4 45.256 0.054 367 49.47 0.053 59.3 59.145 0.045 75.9 59.628 0.05968.5 65.449 0.061 83.5 66.369 0.058 63.2 69.709 0.063 45.1 78.507 0.05811.9 78.739 0.065 11.6 79.363 0.064 15.2 79.787 0.073 5.44 82.276 0.05780.6 83.96 0.073 29.3 85.807 0.074 21 87.072 0.073 6.97

The peaks suggesting a spinel structure, which were suggested above byXRD on Sample 2, were not clearly observed. As described in Embodiment1, the peaks suggesting a spinel structure are sometimes difficult toobserve because of a strong peak derived from the metal oxide 95.

Example 4

In this example, a positive electrode active material was formed by themethod illustrated in FIG. 2B, that is more specifically the methodillustrated in FIG. 3, and initial charge and discharge characteristics,cycle performance under high voltages, cycle performance under hightemperatures, and continuous charge characteristics were evaluated.

A positive electrode active material formed using the method illustratedin FIG. 3 so that the molar concentrations of nickel, aluminum, andmagnesium can be 0.005, 0.005, and 0.01 when the number of cobalt atomsincluded in lithium cobalt oxide is 1 was referred to as Sample 6. Themixing ratio between magnesium fluoride and lithium fluoride wasLiF:MgF₂=1:3 (molar ratio). The first annealing (S34 in FIG. 3) wasperformed at 900° C. for 20 hours, and the second annealing (S56 in FIG.3) was performed at 850° C. for 10 hours; the annealings were bothperformed in an oxygen atmosphere (flow rate of oxygen gas: 10 L/min).

Magnesium fluoride and lithium fluoride were added to lithium cobaltoxide, and annealing was performed once to obtain a positive electrodeactive material, which was referred to as Sample 7. When the number ofcobalt atoms included in lithium cobalt oxide is 1, the molarconcentration of magnesium was adjusted to 0.005. The mixing ratiobetween magnesium fluoride and lithium fluoride was LiF:MgF₂=1:3 (molarratio). The annealing was performed at 900° C. for 20 hours in an oxygenatmosphere (purging with an oxygen gas was performed on a heatingfurnace before the annealing).

Lithium cobalt oxide which was subjected to neither addition of otherelements nor annealing was referred to as Sample 8 (comparativeexample).

Coin cells were fabricated using the positive electrode active materialsof Sample 6 to Sample 8.

The mixing ratio among the positive electrode active material, theconductive additive, and the binder, the electrolyte, the electrolytesolution, the separator, the positive electrode can, and the negativeelectrode can were similar to those of Example 2.

<Initial Charge and Discharge Characteristics and Cycle Performance atCharge Voltages of 4.60 V, 4.62 V, 4.64 V, and 4.66 V>

Cycle performance of the fabricated coin cells were evaluated at chargevoltages of 4.60 V, 4.62 V, 4.64 V, and 4.66 V. Specifically, the CCCVcharge (100 mA/g, various voltages, termination current: 10 mA/g) andthe CC discharge (100 mA/g, termination voltage: 2.5 V) were repeatedlyperformed at 25° C. and 45° C.

FIG. 31A shows cycle performance at 25° C. and a charge voltage of 4.60V. FIG. 31B shows cycle performance at 45° C. and a charge voltage of4.60 V. FIG. 32A shows cycle performance at 25° C. and a charge voltageof 4.62 V. FIG. 32B shows cycle performance at 45° C. and a chargevoltage of 4.62 V. FIG. 33A shows cycle performance at 25° C. and acharge voltage of 4.64 V. FIG. 33B shows cycle performance at 45° C. anda charge voltage of 4.64 V. FIG. 34A shows cycle performance at 25° C.and a charge voltage of 4.66 V. FIG. 34B shows cycle performance at 45°C. and a charge voltage of 4.66 V.

Table 3 shows initial charge capacities and initial discharge capacitiesat 25° C. and 45° C. at the charge voltages. The capacities per weightof the active material are shown in the unit of mAh/g.

TABLE 3 Charge voltage 4.60 V 4.62 V 4.64 V 4.66 V 25° C. Sample 6 220.3220.5 224.2 224.9 charging Sample 7 225.3 227.6 231.4 235.8 Sample 8232.6 235.6 238.6 242.4 45° C. Sample 6 230.4 231.2 234.7 237.0 chargingSample 7 235.8 242.2 247.7 258.6 Sample 8 235.4 241.4 243.7 246.3 25° C.Sample 6 211.5 209.8 213.4 213.4 discharging Sample 7 220.2 221.9 225.7229.2 Sample 8 219.0 217.2 222.7 224.2 45° C. Sample 6 224.2 222.7 225.9227.4 discharging Sample 7 231.1 235.1 240.0 248.1 Sample 8 216.7 218.3205.5 192.4

As is apparent from FIG. 31 to FIG. 34, Sample 6 and Sample 7 showedexcellent cycle performance compared with that of Sample 8, which wassubjected to neither addition of other elements nor annealing. AlthoughSample 6 and Sample 7 showed any significant difference at 25° C. and acharge voltage of 4.6 V, Sample 6 tended to have more favorable cycleperformance at a higher temperature and higher voltages.

It was revealed in Table 3 that Sample 8 had a large irreversiblecapacity due to the initial charge and discharge. The irreversiblecapacity tended to increase at a higher temperature and higher voltages.In contrast, Sample 6 and Sample 7 showed favorable performance withlittle irreversible capacity.

<Cycle Performance at 25° C., 45° C., 50° C., 55° C., and 60° C.>

Next, cycle performance of the coin cells fabricated using the positiveelectrode active materials of Sample 6 to Sample 8 at 25° C., 45° C.,50° C., 55° C., and 60° C. was evaluated. Specifically, the CCCV charge(100 mA/g, 4.6 V, termination current: 10 mA/g) and the CC discharge(100 mA/g, termination voltage: 2.5 V) were repeatedly performed at thetemperatures.

FIG. 35A shows cycle performance of Sample 6 at the temperatures. FIG.35B shows 1st charge and discharge curves and 50th charge and dischargecurves at 50° C. and a charge voltage of 4.60 V. FIG. 36A shows cycleperformance of Sample 7 at the temperatures. FIG. 36B shows 1st chargeand discharge curves and 50th charge and discharge curves at 50° C. anda charge voltage of 4.60 V. FIG. 37A shows cycle performance of Sample 8at the temperatures. FIG. 37B shows 1st charge and discharge curves and50th charge and discharge curves at 50° C. and a charge voltage of 4.60V.

As is apparent from FIG. 35 to FIG. 37, Sample 6 and Sample 7 showedfavorable cycle performance compared with that of Sample 8 at each ofthe temperatures. Sample 6 including nickel and aluminum showedespecially excellent cycle performance at 25° C., 45° C., and 50° C. Asshown by the discharge curves in FIG. 35B, high discharge voltage wasmaintained even at a charge voltage of 4.60 V and 50° C.

At a charge voltage of 4.6 V and 45° C., Sample 6 had an initialdischarge capacity of 220.0 mA/g and a discharge capacity at the 50thcycle of 204.0 mA/g, and the decrease rate of the discharge capacity wasas favorable as lower than 8%. At a charge voltage of 4.6 V and 50° C.,the initial discharge capacity was 223.1 mA/g and the discharge capacityat the 50th cycle was 191.9 mA/g; thus, the decrease rate of thedischarge capacity was as favorable as lower than 14%. Here, thedecrease rate is a value representing an amount of discharge capacitydecreased from the first cycle to a certain cycle with the dischargecapacity at the first cycle being 100%.

<Continuous Charge Test>

Next, a continuous charge test was performed using the coin cellsfabricated using the positive electrode active materials of Sample 6 toSample 8. The continuous charge test is a test for evaluating stabilityand safety of a secondary battery by continuously charging the batteryat a constant voltage for a long time.

One cycle of charge and discharge was performed first, and thencontinuous charge was performed. In the initial charge and discharge,the CCCV charge (38 mA/g, 4.5 V, termination current: 4 mA/g) and the CCdischarge (38 mA/g, termination voltage: 3.0 V) were performed at 25° C.In the continuous charge, the CCCV charge (96 mA/g, 4.60 V, 4.62 V, 4.64V, or 4.66 V) was performed at 60° C. The measurement period of the testwas 250 hours.

FIG. 38A to FIG. 38C show the results at a voltage of 4.60 V; thehorizontal axis represents charging time and the vertical axisrepresents voltage and current. FIG. 38A, FIG. 38B, and FIG. 38C showthe results of the continuous charge test on Sample 6, Sample 7, andSample 8, respectively.

FIG. 39A to FIG. 39C show the results at a voltage of 4.62 V; thehorizontal axis represents charging time and the vertical axisrepresents voltage and current. Similarly, FIG. 39A, FIG. 39B, and FIG.39C show the results of the continuous charge test on Sample 6, Sample7, and Sample 8, respectively.

FIG. 40A to FIG. 40C show the results at a voltage of 4.64 V; thehorizontal axis represents charging time and the vertical axisrepresents voltage and current. Similarly, FIG. 40A, FIG. 40B, and FIG.40C show the results of the continuous charge test on Sample 6, Sample7, and Sample 8, respectively.

FIG. 41A to FIG. 41C show the results at a voltage of 4.66 V; thehorizontal axis represents charging time and the vertical axisrepresents voltage and current. Similarly, FIG. 41A, FIG. 41B, and FIG.41C show the results of the continuous charge test on Sample 6, Sample7, and Sample 8, respectively.

As shown in FIG. 38 to FIG. 41, Sample 8, which was a comparativeexample, showed comparatively stable continuous charge characteristicsat 4.6 V but showed a current increase that is probably derived fromshort circuit within 200 hours at 4.62 V or higher.

In contrast, Sample 6 showed more stable continuous chargecharacteristics as the voltage becomes higher, revealing a high level ofsafety even under conditions of a high temperature of 60° C. and highvoltages of 4.62 V or higher.

Next, the endurance time of the secondary batteries were measured fromFIG. 38 to FIG. 41. The endurance time is described using FIG. 42. InFIG. 42, the horizontal axis represents charging time and the verticalaxis represents voltage and current.

Endurance time TE was obtained by subtracting CC charge finish timeT_(F) from short-circuit time T_(S). Short-circuit time T_(S) was thetime at a point P of intersection of an approximate line L1 for a periodwhen a low current is stably maintained after the start of the CV chargeand an approximate line L2 for a period when a current increase that isprobably derived from short circuit occurs.

Table 4 shows the endurance time of Sample 6 to Sample 8. The unit istime.

TABLE 4 Charge voltage 4.6 V 4.62 V 4.64 V 4.66 V Sample 6205 >250 >250 >250 Sample 7 101 101 106 140 Sample 8 198 192 130 108

As shown in Table 4, the endurance time of Sample 6 was over 200 hoursin every condition. In particular, the endurance time at charge voltagesof 4.62 V, 4.64 V, and 4.66 V was over 250 hours, showing an extremelyhigh level of safety even at a high temperature and high voltages.

REFERENCE NUMERALS

SW1: switch, SW2: switch, SW3: switch, 78 i: current, 81: mixture, 91:material, 92: material, 93: material, 94: material, 95: metal oxide,100: positive electrode active material, 210: electrode stack, 211 a:positive electrode, 211 b: negative electrode, 212 a: lead, 212 b: lead,214: separator, 215 a: bonding portion, 215 b: bonding portion, 217:fixing member, 250: secondary battery, 251: exterior body, 261: foldedportion, 262: seal portion, 263: seal portion, 271: crest line, 272:trough line, 273: space, 300: secondary battery, 301: positive electrodecan, 302: negative electrode can, 303: gasket, 304: positive electrode,305: positive electrode current collector, 306: positive electrodeactive material layer, 307: negative electrode, 308: negative electrodecurrent collector, 309: negative electrode active material layer, 310:separator, 400: glasses-type device, 400 a: frame, 400 b: displayportion, 401: headset-type device, 401 a: microphone portion, 401 b:flexible pipe, 401 c: earphone portion, 402: device, 402 a: housing, 402b: secondary battery, 403: device, 403 a: housing, 403 b: secondarybattery, 405: watch-type device, 405 a: display portion, 405 b: beltportion, 406: belt-type device, 406 a: belt portion, 406 b: wirelesspower feeding and receiving portion, 500: secondary battery, 501:positive electrode current collector, 502: positive electrode activematerial layer, 503: positive electrode, 504: negative electrode currentcollector, 505: negative electrode active material layer, 506: negativeelectrode, 507: separator, 508: electrolyte solution, 509: exteriorbody, 510: positive electrode lead electrode, 511: negative electrodelead electrode, 600: secondary battery, 601: positive electrode cap,602: battery can, 603: positive electrode terminal, 604: positiveelectrode, 605: separator, 606: negative electrode, 607: negativeelectrode terminal, 608: insulating plate, 609: insulating plate, 611:PTC element, 612: safety valve mechanism, 613: conductive plate, 614:conductive plate, 615: module, 616: wiring, 617: temperature controldevice, 900: circuit board, 902: mixture, 903: mixture, 904: mixture,905: mixture, 906: mixture, 907: mixture, 908: mixture, 909: mixture,910: label, 911: terminal, 912: circuit, 913: secondary battery, 914:antenna, 915: antenna, 916: layer, 917: layer, 918: antenna, 920:display device, 921: sensor, 922: terminal, 930: housing, 930 a:housing, 930 b: housing, 931: negative electrode, 932: positiveelectrode, 933: separator, 950: wound body, 951: terminal, 952:terminal, 980: secondary battery, 981: film, 982: film, 993: wound body,994: negative electrode, 995: positive electrode, 996: separator, 997:lead electrode, 998: lead electrode, 7100: portable display device,7101: housing, 7102: display portion, 7103: operation button, 7104:secondary battery, 7200: portable information terminal, 7201: housing,7202: display portion, 7203: band, 7204: buckle, 7205: operation button,7206: input output terminal, 7207: icon, 7300: display device, 7304:display portion, 7400: mobile phone, 7401: housing, 7402: displayportion, 7403: operation button, 7404: external connection port, 7405:speaker, 7406: microphone, 7407: secondary battery, 7500: electroniccigarette, 7501: atomizer, 7502: cartridge, 7504: secondary battery,8000: display device, 8001: housing, 8002: display portion, 8003:speaker portion, 8004: secondary battery, 8021: charging apparatus,8022: cable, 8024: secondary battery, 8100: lighting device, 8101:housing, 8102: light source, 8103: secondary battery, 8104: ceiling,8105: wall, 8106: floor, 8107: window, 8200: indoor unit, 8201: housing,8202: air outlet, 8203: secondary battery, 8204: outdoor unit, 8300:electric refrigerator-freezer, 8301: housing, 8302: door forrefrigerator compartment, 8303: door for freezer compartment, 8304:secondary battery, 8400: automobile, 8401: headlight, 8406: electricmotor, 8500: automobile, 8600: scooter, 8601: side mirrors, 8602:secondary battery, 8603: direction indicator, 8604: under-seat storage,9600: tablet terminal, 9625: switch, 9626: switch, 9627: switch, 9628:operation switch, 9629: fastener, 9630: housing, 9630 a: housing, 9630b: housing, 9631: display portion, 9631 a: display portion, 9631 b:display portion, 9633: solar cell, 9634: charge and discharge controlcircuit, 9635: power storage unit, 9636: DCDC converter, 9637:converter, 9640: movable portion

1. A method for forming a positive electrode active material, the methodcomprising steps of: forming a first mixture in which a first material,a second material, and a third material are mixed; heating the firstmixture to form a second mixture; forming a third mixture in which thesecond mixture, a fourth material, and a fifth material are mixed; andheating the third mixture to form a fourth mixture, wherein the firstmaterial is a halogen compound including an alkali metal, wherein thesecond material includes magnesium, wherein the third material is ametal oxide including the alkali metal and cobalt, wherein the fourthmaterial includes nickel, wherein the fifth material includes aluminum,wherein the heating of the third mixture is performed in a treatmentchamber of an annealing apparatus, wherein a total amount of the thirdmixture heated in the treatment chamber is more than or equal to 15 g,wherein the heating of the first mixture is performed in an atmosphereincluding oxygen, wherein the heating of the first mixture is performedin a temperature range higher than or equal to 600° C. and lower than orequal to 950° C. for more than or equal to 1 hour and less than or equalto 100 hours, wherein the heating of the third mixture is performed inan atmosphere including oxygen, wherein the heating of the third mixtureis performed in a temperature range higher than or equal to 600° C. andlower than or equal to 950° C. for more than or equal to 1 hour and lessthan or equal to 100 hours, and wherein a temperature of the heating ofthe third mixture is lower than a temperature of the heating of thefirst mixture by 20° C. or more.
 2. The method for forming a positiveelectrode active material according to claim 1, wherein the alkali metalis lithium, wherein the first material is lithium fluoride, and whereinthe second material is magnesium fluoride.
 3. The method for forming apositive electrode active material according to claim 1, wherein thethird material is nickel hydroxide, and wherein the fourth material isaluminum hydroxide.
 4. A method for forming a positive electrode activematerial, the method comprising steps of: forming a first mixture inwhich a first material, a second material, a third material, and afourth material are mixed; and heating the first mixture to form asecond mixture, wherein the first material is a halogen compoundincluding an alkali metal, wherein the second material includesmagnesium, wherein the third material includes one or more selected fromnickel, aluminum, titanium, vanadium, and chromium, wherein the fourthmaterial is a metal oxide including the alkali metal and cobalt, whereinthe heating of the first mixture is performed in a temperature rangehigher than or equal to 600° C. and lower than or equal to 950° C. formore than or equal to 1 hour and less than or equal to 100 hours,wherein when a third mixture is formed by mixing the first material, thesecond material, and the third material and the third mixture issubjected to differential scanning calorimetry, the third mixture has afirst peak having a local minimum value in a range higher than or equalto 620° C. and lower than or equal to 920° C., and wherein the firstpeak is a negative peak.
 5. The method for forming a positive electrodeactive material according to claim 4, wherein the alkali metal islithium, wherein the first material is lithium fluoride, and wherein thesecond material is magnesium fluoride.
 6. The method for forming apositive electrode active material according to claim 4, wherein thethird material includes nickel, wherein the first mixture is a mixturein which a fifth material is mixed with the first material, the secondmaterial, the third material, and the fourth material, and wherein thefifth material includes aluminum.
 7. The method for forming a positiveelectrode active material according to claim 6, wherein the thirdmaterial is nickel hydroxide.
 8. The method for forming a positiveelectrode active material according to claim 4, wherein a half width ofthe first peak is lower than 100° C.
 9. The method for forming apositive electrode active material according to claim 4, wherein ameasurement temperature range of the differential scanning calorimetryat least includes a range higher than or equal to 200° C. and lower thanor equal to 850° C.
 10. The method for forming a positive electrodeactive material according to claim 4, wherein the heating of the firstmixture is performed in an atmosphere includes oxygen.
 11. A secondarybattery comprising: a positive electrode active material, wherein thepositive electrode active material includes lithium, cobalt, magnesium,and fluorine, and wherein when 50 cycles of charge and discharge areperformed at a charge voltage of 4.60 V at 50° C., an amount of adischarge capacity decreased from a first cycle to a 50th cycle is lowerthan or equal to 14% with a discharge capacity at the first cycle being100%.
 12. The secondary battery according to claim 11, wherein thepositive electrode active material includes nickel and aluminum.